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Spatial and Bidirectional Work Function Modulation of Monolayer Graphene with Patterned Polymer “Fluorozwitterists”
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Cite this: ACS Cent. Sci. 2024, 10, 8, 1629–1639
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https://doi.org/10.1021/acscentsci.4c00704
Published August 6, 2024

Copyright © 2024 The Authors. Published by American Chemical Society. This publication is licensed under

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Abstract

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Understanding the electronic properties resulting from soft–hard material interfacial contact has elevated the utility of functional polymers in advanced materials and nanoscale structures, such as in work function engineering of two-dimensional (2D) materials to produce new types of high-performance devices. In this paper, we describe the electronic impact of functional polymers, containing both zwitterionic and fluorocarbon components in their side chains, on the work function of monolayer graphene through the preparation of negative-tone photoresists, which we term “fluorozwitterists.” The zwitterionic and fluorinated groups each represent dipole-containing moieties capable of producing distinct surface energies as thin films. Kelvin probe force microscopy revealed these polymers to have a p-doping effect on graphene, which contrasts the work function decrease typically associated with polymer-to-graphene contact. Copolymerization of fluorinated zwitterionic monomers with methyl methacrylate and a benzophenone-substituted methacrylate produced copolymers that were amenable to photolithographic fabrication of fluorozwitterist structures. Consequently, spatial alteration of zwitterion coverage across graphene yielded stripes that resemble a lateral p-i-n diode configuration, with local increase or decrease of work function. Overall, this polymeric fluorozwitterist design is suitable for enabling simple, solution-based surface patterning and is anticipated to be useful for spatial work function modulation of 2D materials integrated into electronic devices.

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Synopsis

The fluorozwitterist macromolecular design enables the use of a single-component polymer overlayer for simple photopatterning of the work function of graphene to resemble a p-i-n diode configuration.

Introduction

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Work function (WF) engineering of two-dimensional (2D) materials constitutes an effective route to enhanced performance of electronic devices. (1) The WF of a conducting material, defined as the energy required to promote an electron from Fermi level to vacuum, can be tailored to facilitate charge injection, alter band alignment, and, for 2D materials, control the nature and extent of doping. (2−4) For monolayer graphene, strategies to modulate WF include chemical modification, application of mechanical strain, and electrostatic gating. (5−9) Another prominent approach involves synthetic polymers, which are attractive for their solution processability, chemical functionality, film-forming capabilities, and utility in lithographic patterning. (10−12) In particular, polymer zwitterions, which possess pendent groups with covalently connected cationic and anionic components, are recognized as useful for their orthogonal solubility (advantageous for layer-by-layer deposition), electrical neutrality (lacking mobile counterions that may interrupt stability and performance), and antifouling behavior (reducing protein adsorption that can degrade bioelectronic devices). (13−15) Our prior studies uncovered the substantial WF shifts of metal electrodes and 2D materials induced by polymer zwitterions, which led to enhanced performance in field-effect transistors and solar cells, (16−18) where electronic response arises from the dipole moments associated with the physisorbed zwitterions. (19)
In earlier studies on photopatternable polymer zwitterions on graphene, the introduction of sterically bulky groups, such as piperidine, on the ammonium cation of the sulfobetaine (SB) zwitterion led to enhanced WF reduction (i.e., n-type doping). (20) Density functional theory (DFT) calculations pointed to the importance of the normal vs. transverse components of the zwitterion dipole relative to the underlying graphene layer (noting that the WF shift is directly proportional to the normal component of the dipole) and that the steric footprint of piperidine may orient the SB cation away from the underlying graphene and toward surface normal orientation. That is, polymers with pendent zwitterions oriented normal to graphene produce larger WF shifts than those with transverse orientations, which agreed qualitatively with experimental results obtained by ultraviolet photoelectron spectroscopy (UPS). Further, owing to the presence of benzophenone-containing comonomers, these zwitterionic polymers proved useful as negative-tone photoresists, or “zwitterists”, amenable to photo-cross-linking in areas of UV-exposure and fabrication of graphene-based field-effect transistor (FET) devices. We subsequently probed the electronic influence of other polymer zwitterions in contact with graphene, such as poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC). (21) Scanning probe evaluation of both the polymer and graphene sides of the polymer-on-graphene construct revealed that polarization, rather than pure charge transfer, is principally responsible for graphene doping. Although the zwitterionic dipoles of PMPC are oriented in a direction opposite to that of the SB-methacrylate polymer backbone, both structures led to n-doping, noting that calculations inferred that steric factors of the cationic moieties of the pendent PC groups push them away from the surface. Overall, these prior studies set forth exciting opportunities for WF modulation through variation of zwitterion chemical structure that may alter molecular orientation.
The chemical versatility of polymers encourages tethering of other dipole-rich and surface-active functionalities, beyond hydrocarbons, to modulate electronic properties. For example, we successfully embedded fluorocarbons directly into choline phosphate (CP) monomers, which upon polymerization afforded fluorinated choline phosphate (FCP)-based polymer zwitterions. (22) Surface grafting of FCP polymers revealed contact angles and surface energies intermediate between those of PMPC and conventional fluorinated polymers, (23) with large contact angle hysteresis values pointing to dynamic reorganization in response to the contacting fluid. Given the electron-withdrawing character and control of molecular orientation imparted by the fluorocarbons, we sought to investigate the potential for electronic interactions resulting from physisorption of FCP polymers on graphene. The strong orientational driving forces associated with the surface energy of fluorinated groups (24,25) and their reported role as p-dopants of 2D materials (26,27) further motivate the studies described below.
We specifically describe the electronic impact of polymeric FCPs on the WF of monolayer graphene to generate negative-tone “fluorozwitterists.” Notably, these unusual polymer zwitterions have two types of dipoles embedded in their side chains, each with distinct contributions to surface energy: one arising from the zwitterionic CP groups and the other from the fluorinated alkyl groups. Kelvin probe force microscopy (KPFM) of FCP-coated graphene revealed electronic characteristics indicative of p-doping of graphene. Sum-frequency generation (SFG) vibrational spectroscopy provided chemical and structural insights into how the dipoles orient at the polymer–graphene interface. Moreover, the preparation and use of FCP-containing copolymers with methyl methacrylate (MMA) and a benzophenone-substituted methacrylate (BPMA) gave a route to robust film formation and photo-cross-linking, both valuable features for lithographic patterning. By patterning FCP polymers with PSBMA on the same graphene substrate via sequential UV-lithography, neighboring zwitterionic stripes were prepared that resemble a p-type/intrinsic/n-type (p-i-n) diode configuration laterally across graphene. Overall, the knowledge gained from this study holds promise for fine-tuning the electronic characteristics of 2D materials-based devices, including the spatial patterning of p- or n-type character by simple contact with dipole-rich moieties.

Results and Discussion

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Polymer Synthesis and Electronic Characterization

The functional polymers utilized in lithographic patterning and fundamental investigations were synthesized by controlled free radical polymerization of FCP monomers with MMA and BPMA. This yielded polymers amenable to photolithography in a similar fashion as the “zwitterist” macromolecular design (20) by employing a photo-cross-linkable monomer with this new class of fluorinated zwitterions. (22) Specifically, the FCP monomers in Figure 1 were employed in copolymerizations using reversible addition–fragmentation chain transfer (RAFT) polymerization at 70 °C in a 2,2,2-trifluoroethanol (TFE) solution containing a dithiobenzoate chain transfer agent and an azo-initiator. This afforded the desired random copolymers with estimated number-average molecular weight (Mn) in the 11–20 kDa range and polydispersity index (Đ) of 1.1–1.2. Their corresponding 1H NMR spectra (Figure S1 and S3) showed distinct signals from each repeat unit, enabling calculation of polymer composition (50:44:6 and 53:41:6 FCP:MMA:BPMA for a typical example of HFIP-CP and PFO-CP copolymers, respectively) that was in good agreement with the feed ratio employed (50:45:5). In addition, homopolymers of HFIP-CP and PFO-CP were prepared to probe the impacts of the FCP moieties without the influence of MMA and BPMA. To gain insight into the distinct contributions of each dipole type, homopolymers of PMPC and the nonzwitterionic fluorinated methacrylate termed PTDFOMA were also prepared, with the collection of structures shown in Figure 2a (and characterization data given in Figures S5–S8 and Table S1).

Figure 1

Figure 1. Top: Synthesis of fluorinated zwitterion-based random copolymers by RAFT polymerization with variation of fluorinated (F) groups; Bottom: lithographic patterning of “fluorozwitterist” on graphene through a chrome-coated photomask for spatial electronic tuning; Far-right: photograph of actual 1000-μm wide patterned stripes (blue) on graphene/SiO2/Si substrate (purple).

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Figure 2

Figure 2. (a) Chemical structures of homopolymers of 1,1,1,3,3,3-hexafluoro-2-propanol-substituted choline phosphate (HFIP-CP) and 1H,1H,2H,2H-perfluoro-1-octanol-substituted choline phosphate (PFO-CP), poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC), and poly(tridecafluoro-n-octyl methacrylate) (PTDFOMA); (b) Work function distributions derived from KPFM of homopolymers on graphene/SiO2/Si deposited from 1 mg/mL solutions; and (c) Illustration of hypothesized orientation of pendent FCP group with respect to graphene.

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To assess the electronic influence of polymer films on graphene, dual-pass KPFM was employed as a scanning probe technique to measure the electric force between a conductive microcantilever and a grounded sample. (28) KPFM characterizes the impact of the environment on the electronic properties of 2D materials, such as from physisorbed surface dopants. (10,11,20,21,29−31) In dual-pass KPFM, surface topography was determined during the first pass, followed by a second pass to measure surface potential contrast (SPC). Changes in WF are derived from SPC by calibrating the WF of the probe with a highly oriented pyrolytic graphite (HOPG) reference. In our experiments, polymer films on graphene/SiO2/Si substrates were applied by spin-coating from 1 mg/mL TFE solutions at 4000 rpm. Spectroscopic ellipsometry revealed polymer layers with an estimated thickness of ∼3–4 nm (Figure S9). KPFM indicated a marked difference in doping effects imparted by the four homopolymers, as depicted in Figure 2b, with the corresponding height and WF images given in Figure S10. Notably, PMPC was the only polymer to induce work function reduction, by ∼87 meV, which agrees qualitatively with our prior work. (21) In contrast, the fluorine-containing polymers resulted in a WF increase of graphene, indicative of a p-doping effect elicited by the polymer coating. Specifically, PTDFOMA, HFIP-CP, and PFO-CP homopolymers led to WF increases of ∼245, ∼333, and ∼409 meV, respectively. While p-type doping induced by PTDFOMA may be anticipated from the presence of the electron-withdrawing fluorocarbon structure, the FCP homopolymers produced the largest WF shifts despite having fewer (HFIP) or identical (PFO) numbers of fluorine atoms as PTDFOMA. This apparent synergistic electronic action between the zwitterionic and fluorinated groups in contact with graphene inverts the sign of Δϕ relative to PMPC and augments the magnitude relative to PTDFOMA. This finding may be rationalized by considering the potential orientation of the FCPs, where localization of the fluorocarbon components to the air interface drives zwitterion orientation such that the cationic moiety points toward graphene (Figure 2c). In contrast, in our prior work, calculations indicated that the zwitterionic moieties in PMPC and PSBMA prefer to physisorb in a nearly flat orientation on graphene, which maximizes attractive dispersion interactions. (20,21) Thus, we suggest that the competition between the energetics of zwitterion physisorption on graphene and segregation of the fluorocarbon component to the air interface may ultimately control FCP orientation.
While KPFM measures a relative energy difference in work function between a conductive tip and an underlying substrate, UPS directly probes the electronic levels in the valence band by measuring the kinetic energy of emitted photoelectrons following UV absorption under ultrahigh vacuum. (32) Specifically, the energy difference between the incident photons (21.2 eV for He I radiation) and that corresponding to the secondary electron cutoff in the obtained spectra represents the WF shift. To prevent charging (i.e., blocking collection of low kinetic energy electrons upon photoelectron emission), a bias voltage is normally applied, which necessitates use of conductive samples. Here, a graphene/Au/Ti/SiO2/Si configuration was employed as the substrate, where the Au underlayer enhances sample conductivity and minimizes charging. Furthermore, since the typical information depth for UPS is 2–3 nm, very thin polymer films were prepared from 0.1–0.5 mg/mL polymer solutions on Au-coated wafers (measured as <2 nm by spectroscopic ellipsometry) (Figure S9). These thickness values agree with the step-heights obtained from atomic force microscopy (AFM) measurements performed on scratched polymer-coated graphene/SiO2/Si samples (Figure S11).
As shown in Figure 3, polymer film thickness influenced the magnitude of WF shifts (20) relative to bare graphene measured at 4.02 eV (denoted by horizontal line). When coated with PMPC and PTDFOMA, the measured WF shifted to lower and higher values, respectively, in agreement with KPFM measurements. The magnitudes increased with film thickness, with respective shifts of −0.93 and 0.56 eV observed as PMPC and PTDFOMA films approached 2 nm. As such, polymer film thickness provides a tunable knob for electronic modulation as may be desirable for a given application. In particular, the electron-withdrawing groups of PTDFOMA induced an increase in hole density, shifting the Fermi level of graphene closer to the valence band, and increasing the energy required to promote an electron to vacuum. (33) On the other hand, calculations suggest that the anionic component of MPC (with a μ⊥ equal to 0.52 D) points toward graphene, (21) which should decrease the vacuum level and consequently reduce WF, in accord with studies on polymer zwitterion-coated metals (34) and organic–metal interfaces. (35)

Figure 3

Figure 3. Comparison of WF values of polymer-coated graphene measured by UPS (mean ± SD of n = 4 to 10 replicates).

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Both HFIP-CP and PFO-CP coatings produced WF increases, especially for thinner films, with magnitudes comparable to PTDFOMA, confirming the p-doping behavior of FCP polymers. Interestingly, for the thickest films shown in Figure 3, WF shifts were smaller, which we attribute to charging (accumulation of surface charge upon electron emission). Very thick films (>10 nm) led to unreasonable WF values upon bias correction (i.e., the correction factor should be less than the applied voltage to set the Fermi level to zero). While at this stage we do not understand the work function finding for HFIP-CP at ∼1.5 nm film thickness (deviating from the other data points), we speculate that the lower degree of fluorination of this polymer may reduce the influence of orientation as film thickness increases. Notably, UPS relies on the kinetic energy of emitted photoelectrons to detect electron energy loss due to work required when passing through the polymer layer. Moreover, although both UPS and KPFM provide information on electronic levels, UPS measures area-average WF while KPFM scans the local WF with high spatial resolution. Thus, UPS is sensitive to surface contamination (imperfections or pinholes), and as such complementary KPFM measurements of thicker films are useful to understand if surface imperfections have impacted UPS measurements. Intrinsic defects that may arise within the lattice structure of graphene can impact the electronic properties and should be considered as well. (36,37) Additional contributions from stacked dipoles, involving multiple layers of zwitterions, may also play a role, though at present we have no evidence to support the formation of such structures. Irrespective of the exact mechanisms responsible for the effects, the electronic characterization demonstrated to this point shows that altering zwitterion chemistry enables control over doping behavior on graphene, from n-type to p-type, and thus in principle sets up a fabrication strategy for in-plane device construction.

Probing Interfacial Configuration by Sum-Frequency Generation Vibrational Spectroscopy

To probe the distribution and packing of chemical moieties adsorbed on graphene, we employed sum-frequency generation (SFG) vibrational spectroscopy. Based on symmetry arguments, SFG signals are interface-specific, allowing one to gain chemical and structural insights into buried interfaces that are otherwise inaccessible to conventional linear spectroscopies. Particularly, we sought to understand the effect of the pendent group functionality on the organization of the polymer backbone when interacting with graphene, and to relate these structural motifs to the change in work function. Details of the measurements are given in the Methods section.
Figure 4 displays the SFG spectra obtained from PMPC, HFIP-CP, PFO-CP, and PTDFOMA homopolymers adsorbed on graphene/SiO2 (quartz) at SSP and PPP polarization combinations. The solid curves correspond to fits to the experimental data points obtained using eq 1 in the Methods section. The data in this frequency region report on CH vibrational modes from the methacrylate backbone and the various pendent arms, as assigned below. Control experiments were performed with thicker polymer films (∼1 μm) deposited on SiO2 to identify signals arising from the air–polymer interface (Figure S12a). The control spectra clearly differ from those obtained from thin (4–5 nm) polymer–graphene (Figure 4) and polymer–SiO2 interfaces (Figure S12b), indicating that the SFG signals for thin polymer coatings correspond to the polymer–substrate interface and that the supporting substrate strongly affects polymer organization at the buried interface.

Figure 4

Figure 4. SFG spectra obtained from PMPC, HFIP-CP, PFO-CP, and PTDFOMA polymer thin films deposited on graphene/SiO2 (quartz) at (a) SSP and (b) PPP polarization combinations. The spectra are offset for clarity.

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In all SSP spectra presented in Figure 4a, the prominent feature at 2900 cm–1 is assigned to the symmetric stretch (-ss) from the methyl group (−CH3) on the methacrylate chain. (38−40) The broad peak at 2950 cm–1 in the SSP spectra of both HFIP-CP and PMPC is attributed to a Fermi resonance with unresolved contributions from out-of-phase −CH3 asymmetric stretches (-as). (38,40) These asymmetric stretching modes are evident in the PPP spectra (Figure 4b), in agreement with SFG selection rules. Additionally, the PMPC SSP spectrum shows broad unresolved features at lower wavenumbers (2845 to 2865 cm–1) that arise from methylene symmetric stretches in the methacrylate backbone (C–CH2) and the pendent arms (O–CH2/N–CH2). (38,39) These CH2 bands are not detected in the SSP spectra from other polymers, indicating that (1) the presence of fluorinated groups in the pendent arms influences the overall methacrylate backbone conformation and (2) the pendant arms themselves orient in unique ways depending on the chemical makeup. Supporting this, we identified C–CH2-as (2895 cm–1) and weaker O–CH2-as/N–CH2-as (2915 cm–1) modes in the PPP spectra of HFIP-CP and PFO-CP polymers, respectively. (38)
To further corroborate the qualitative structural differences observed with different pendant groups, control experiments were performed in which the polymers were deposited on SiO2 coverslips without graphene (Figure S12b). Here, the polymer chains contact a polar surface environment (in contrast to the nonpolar graphene or air), which should result in distinct interfacial conformations. Indeed, the spectral profiles at SiO2 interfaces show clear differences compared to those at graphene interfaces in Figures 4 and S12a, indicating unique interactions of the pendant arms with the polar SiO2 surface and different associated backbone conformations. Interestingly, all spectra from SiO2 interfaces have observable contributions from CH2 groups (both backbone and pendent groups), whereas the prominent −CH3-ss resonance found in graphene samples appears as a weak shoulder. This means that the polymer backbone and pendent groups arrange differently at graphene vs. polar interfaces, thereby permitting modulation of the work function by differential packing and associated dipolar couplings. (41) For instance, at graphene interfaces, PTDFOMA polymers with longer fluorinated chains result in a poorly ordered interface, as evidenced by weak overall SFG signals (Figure 4). (42−44) Similarly, the PFO-CP samples exhibit poor overall signal and ordering, indicating that longer pendant arms with increasing fluorine content generally disrupt interfacial polymer packing. We note that the interfacial ordering at graphene interfaces, as qualitatively gauged by changes in SFG response, does not map one-to-one onto the measured work function shifts. That is, while interfacial order goes as HFIP-CP > PMPC > PFO-CP ∼ PTDFOMA, only PMPC was found to reduce the work function of graphene.
To more quantitatively elucidate the impact of orientation on work function, we performed numerical analysis, as previously reported, (38,45,46) with the parameters used detailed in the Supporting Information. We did not obtain any orientational information for PTDFOMA, due to the weak signals in the PPP polarization combination. Taking the CH3-as,PPP/CH3-ss,SSP amplitude ratios for the other three polymers, the average tilt angle of the −CH3 group of the methacrylate backbone with respect to the surface normal is in the range of 31–37° for PMPC and FCP, as represented by θ in Figure 5. These results suggest that the fundamental interactions between the graphene monolayer and the nonpolar backbone remain unchanged when varying pendant arms, but that the interfaces become increasingly disordered.

Figure 5

Figure 5. Schematic diagram of the hypothesized orientations of (a) PMPC and (b) FCP polymer chains adsorbed on graphene based on SFG findings. For clarity, methylene groups in the pendant arm are drawn in brown. Pendant arm orientations with respect to graphene corroborate the measured work function shifts.

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Similarly, we can interpret the arrangement of the CH2 groups in the pendant arm by noting their intensities; here, the CH2 groups on the PMPC pendent arms are generally tilted away from the surface plane, thus resulting in stronger peaks in the SSP spectra. This results in a whole pendant arm orientation that is parallel to the surface plane, as illustrated in Figure 5a. In contrast, for HFIP-CP and PFO-CP polymers, the lack of this signal from CH2 groups suggests that these groups are oriented parallel to the surface plane, inducing the whole pendant arm to stand more upright in a bottlebrush-like configuration, as shown in Figure 5b. The different orientations of the pendant arms should result in different dipolar couplings between the zwitterion and graphene. Those polymers with fluorinated tails correspond to systems with larger work functions, where the zwitterion orientation is likely to be aligned out of the interfacial plane. For PMPC, the only sample that produced a decrease in work function, the pendant arms orient such that the zwitterion would be more closely parallel to the interface, in agreement with calculations. (21) Based on the SFG results and relating them to the hypothesized orientation of pendent arms illustrated in Figure 2c, we find that the presence of a longer fluorinated side chain results in poorer backbone packing, presumably mediated by pendant arms. These pendant groups containing the zwitterionic moiety then have different couplings with graphene based on their absolute orientation with respect to the surface.

Lithographic Patterning of Fluorozwitterists

The prepared copolymers of fluorinated zwitterions with MMA and BPMA were then utilized as negative-tone photoresists or “fluorozwitterists.” Polymer solutions in TFE were similarly spin-coated on graphene/SiO2/Si substrates, yielding films of 20–30 nm thickness. Figure 1 illustrates the photolithographic process for patterning fluorozwitterists on graphene, accomplished by UV irradiation at 365 nm with an energy dose of 20,000 mJ cm–2 through a chrome-coated quartz photomask, followed by developing in TFE to remove un-cross-linked regions, and drying under N2(g). Lithographic patterning was successful on films prepared from 5 mg/mL polymer solutions; employing more dilute solutions led to either no stripe formation or the appearance of thin, nonuniform features (Figure S14). Both random copolymers produced uniform stripes of 20–30 nm thickness, with pitch of ∼50 μm, as shown in Figure 6, indicating good pattern fidelity relative to the mask employed. Dual-pass KPFM enabled topographic and electronic evaluation of patterned polymer–graphene interfaces (Figure 6b,c,f,g). Although both stripes comprised only ∼50 mol % of the fluorinated zwitterions, the p-doping behavior was evident. A sharp WF increase over a distance of ∼1–2 μm was quantified across the graphene interfaces with both fluorozwitterists (Figure 6d,h). Specifically, the patterned HFIP-CP and PFO-CP, with RMS surface roughness of ∼2 and ∼15 nm, induced WF shifts of ∼50 and ∼120 meV, respectively, in qualitative agreement with the WF shifts for the corresponding homopolymers. Despite the roughness of the PFO-CP stripe, the WF changes remained smooth. Notably, the unpatterned PFO-CP homopolymer did not exhibit such surface roughness (Figure S10) and any roughness observed is not due to polymer crystallinity, as differential scanning calorimetry confirmed their amorphous nature (Figure S15). Thus, surface roughness likely resulted from the lithographic process.

Figure 6

Figure 6. (a,e) Optical microscopy of HFIP-CP and PFO-CP copolymers with 50 μm patterned stripes on graphene/SiO2/Si substrate. Kelvin probe force microscopy of the interface between patterned stripes and bare graphene showing the (b,f) height and (c,g) work function images with corresponding (d,h) line profiles. Both polymers induced p-doping relative to bare graphene surface.

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Co-patterning of Fluorozwitterist and Zwitterist

Considering our findings to this point, we hypothesized that graphene doping may be spatially modulated by lateral patterning of different polymer zwitterions. To test this, a random copolymer of SBMA with MMA and BPMA (Figure S16) was employed as the solution-processable zwitterist. Figure 7 exhibits the simple process for copatterning distinct polymer zwitterions via masked UV lithography. First, a solution of the FCP-based random copolymer was spin-coated on a graphene/SiO2/Si substrate. The sample was dried under vacuum at 40 °C prior to photo-cross-linking at 365 nm with a dose of 20,000 mJ cm–2 through a chrome-coated quartz photomask. The mask was positioned so as to block a region of the first coating from being cross-linked and provide space for the next polymer zwitterion to be patterned. After UV exposure, the wafer was soaked in TFE for 10 s to remove the un-cross-linked regions and dried with a stream of N2(g). Then, a second solution containing PSBMA copolymer was spin-coated on the partially patterned graphene/SiO2/Si substrate. Repeating the exposure and development procedures yielded neighboring stripes of fluorozwitterist and zwitterist. X-ray photoelectron microscopy (XPS) analysis revealed excellent fidelity of the patterned regions, in which the expected elements were observed in the stripes of FCP (without sulfur contamination) and PSBMA (without fluorine or phosphorus contamination).

Figure 7

Figure 7. Top: Schematic diagram for copatterning polymeric fluorozwitterist (FCP) and zwitterist (PSBMA) on the same graphene substrate via sequential UV lithography; Bottom: XPS survey spectra of individual patterned polymer stripes (blue) on graphene (purple) indicated on the photograph with relevant peaks corresponding to the dipole-doping moieties.

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KPFM measurements validated our hypothesis and uncovered local WF shifts on the same substrate relative to nearby bare graphene regions induced by the FCP (WF increase) and PSBMA (WF decrease). As seen in Figure 8, the HFIP-CP and PSBMA copolymer patterns correspondingly shifted the WF of graphene by 75 meV (p-doping) and −70 meV (n-doping), wherein the neighboring stripes resemble a lateral p-i-n diode configuration. This structure has increasingly gained attention for low-power, high-speed optoelectronics, wherein the intrinsic region induces a lateral built-in electric field that is essential for ultrafast and efficient separation of photogenerated carriers. (47,48) The p-i-n configuration is useful for expanding the utility of graphene, which inherently has a low on/off current ratio due to the absence of a bandgap. Common approaches to spatially control WF of graphene include substitutional doping, (49) patterned coating, (50) and layering. (51) A strategy more conceptually related to our method involve patterning of self-assembled monolayers (SAMs), wherein CH3-terminated SAMs neutralized the p-doping behavior of the SiO2 substrate while NH2-terminated SAMs n-doped graphene. (6) However, this requires silane coupling agents and a photoresist to grow and pattern SAMs on Si, followed by pattern transfer, whereas the fluorozwitterist design simplifies the fabrication process since the polymer acts simultaneously as the coating, resist, and dopant, thus bypassing the etching and transfer steps.

Figure 8

Figure 8. Co-patterned HFIP-CP (fluorozwitterist) and PSBMA (zwitterist) copolymers: KPFM of the interface between patterned stripes and bare graphene showing the (a,d) height and (b,e) work function images with corresponding (c,f) line profiles. The patterned fluorozwitterist and zwitterist induced p- and n-doping, respectively, relative to neighboring bare graphene surface.

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We also observed the importance of the patterning order: thinner films of the fluorozwitterists with larger electronic shifts were realized when FCP was patterned first, followed by PSBMA. In contrast, the WF shifts were less significant when FCP patterning followed PSBMA (Figures S17–19 and Table S2). Since XPS, which probes the stripes to ∼10 nm depth, provided evidence for the generation of discrete polymer stripes, the impact of patterning order may suggest some degree of FCP-PSBMA mixing at the graphene–polymer interface, though we have not probed this specifically. Nonetheless, the patternability of these polymers provides an opportunity to produce devices with two laterally adjacent or remote polymers in contact with the 2D material to elicit local surface potential modification and form homojunctions while preserving the structural and electronic properties of graphene. Investigating the electronic performance of copatterned graphene devices is central to our future work. The simple, solution-based lithographic strategy we introduced herein holds great potential for spatially controlling the nature and extent of doping on 2D surfaces with arbitrarily predetermined patterns and promising lateral resolution.

Conclusions

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We demonstrated p-doping of graphene by its interfacial contact with fluorinated polymer zwitterions and provided fundamental insights into the role of zwitterionic dipole orientation. Interestingly, the WF shift induced by the fluorinated polymer zwitterions surpassed the shifts caused by a more conventional methacrylic fluoropolymer. By simple solution-based copatterning of different polymer zwitterions, both positive and negative WF shifts were measured on the same graphene substrate, conceptually resembling a lateral p-i-n diode configuration. The knowledge gained from this study provides insights for the field of 2D materials-based devices by elucidating the importance of tailored hard–soft 2D-polymer interfaces, in which polymer chemistry is combined with patterning methodology to afford access to spatially controlled doping of 2D electronic structures.

Methods

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Polymer Synthesis and Characterization

Copolymers of FCP-substituted methacrylate, MMA and BPMA were prepared by reversible addition–fragmentation chain-transfer (RAFT) polymerization as shown in Figure 1. The chain transfer agent 4-cyano-4-(phenylcarbonothioylthio)pentanoic acid, 4,4-azobis(4-cyanovaleric acid) (ACVA) initiator ([CTA]:[ACVA] = 1:0.2), FCP monomer (50 equiv), MMA (45 equiv), BPMA (5 equiv) and TFE (1.6 M with respect to total monomer amount) were mixed in a 7 mL vial wrapped with aluminum foil. The reaction mixture was degassed using dry N2(g) for 30 min and then stirred at 70 °C for 10 h. The polymerization was quenched by removal from heat source and exposure to air. Triple precipitation in diethyl ether, water, and diethyl ether followed by drying in vacuo afforded the solid polymer product (∼50% yield). The homopolymers were synthesized following similar procedures without the addition of MMA and BPMA. 1H, 19F, and 31P NMR (500 MHz) spectra of the polymer products were recorded on a Bruker Ascend 500 spectrometer equipped with a Prodigy cryoprobe. Gel permeation chromatography (GPC, using PMMA calibration standards) was conducted using an eluent mixture of TFE with 0.02 M sodium trifluoroacetate at 40 °C on an Agilent 1200 system equipped with the following: an isocratic pump operated at 1 mL/min, a degasser, an autosampler, one 50 mm × 8 mm PSS PFG guard column (Polymer Standards Service), and three 300 mm × 7.5 mm PSS PFG analytical linear M columns with 7 μm particle size (Polymer Standards Service), and an Agilent 1200 refractive index detector.

Polymer Film Sample Preparation

CVD monolayer graphene was prepared and transferred onto a 4-in. Au/Ti/SiO2/Si wafer by Grolltex, Inc. (San Diego, CA). The substrate was then cut into 1 cm × 1 cm pieces. Solutions of FCP homopolymers as well as PMPC and PTDFOMA with varying concentrations, from 0.1 to 1.0 mg/mL, were individually spin-coated onto these substrates at 500 rpm for 5 s and then at 4000 rpm for 55 s. Prior to deposition, the polymer solutions were vortexed and filtered through a polytetrafluoroethylene (PTFE) membrane (0.2 μm VWR). To remove residual solvent, the film samples were dried under vacuum at 40 °C overnight prior to electronic and spectroscopic characterizations.

Lithographic Patterning

Solutions of FCP-based random copolymers in TFE with an optimal concentration of 5 mg/mL were individually spin-coated onto graphene/SiO2/Si substrates at 500 rpm for 5 s and then at 4000 rpm for 55 s. Prior to deposition, the polymer solutions were vortexed and filtered through a PTFE membrane (0.2 μm VWR). To remove residual solvent, samples were dried under vacuum at 40 °C overnight prior to lithography. The polymer films were cross-linked by UV irradiation (λ = 365 nm) with a dose of 20,000 mJ cm–2 through a chrome-coated quartz photomask. The samples were soaked in TFE for 10 s to remove the un-cross-linked regions and dried with a nitrogen gun, affording patterned resists. For copatterned resists, the same procedure was performed twice with 4 mg/mL of PSBMA copolymer in TFE as the additional polymer solution. Optical imaging of patterned samples was conducted using a ZEISS Axioscope 5 microscope with Axiocam 305 color camera.

Polymer Film Thickness Measurement

The same homopolymer solutions as described above, with varying concentrations, were separately spin-coated on Au substrates at 500 rpm for 5 s and then 4000 rpm for 55 s. Polymer film thickness values were estimated by ellipsometry using a J.A. Woollam RC2 spectroscopic ellipsometer at varying angles of incidence (45°, 50°, 55°, 60°, 65°). The values were calculated by fitting the experimental data with the Cauchy equation: n = A + B/λ2 where n is the refractive index, λ is the light wavelength in μm, and A and B are constants with value of 1.5 and 0.01, respectively.

Kelvin Probe Force Microscopy (KPFM)

KPFM data were collected on a Digital Instruments Bioscope AFM/KPFM in two-pass lift mode under ambient atmospheric conditions (22 °C, 45% RH). The AFM probes were platinum/iridium-coated silicon (SCM-PIT-V2) with f0 of ∼70 kHz, used as supplied by Bruker. Samples were grounded using copper tape from Electron Microscopy Sciences. Work function was calculated by measuring a freshly cleaved highly oriented pyrolytic graphite (HOPG, ZYB grade, Bruker) reference sample (with a work function of 4.65 eV) to calibrate the tip’s work function, which was then used to relate the measured surface potential to the corresponding work function. The profiles were analyzed using the scanning probe microscopy data analysis software Gwyddion.

Ultraviolet Photoelectron Spectroscopy (UPS) and X-ray Photoelectron Spectroscopy (XPS)

Both UPS and XPS were carried out using a Thermo Scientific Nexsa Surface Analysis System with a He I discharge line (21.2 eV) and monochromatic aluminum Kα X-ray source (1486.6 eV), respectively. For UPS, a −10 V sample bias was applied to collect the low kinetic energy electrons. All data were obtained with a pass energy of 2.0 V and a step size of 0.05 eV at a base pressure of 5 × 10–7 millibar or lower. The UPS spectra reported were averaged from three scans. For XPS, the flood gun was turned on during all measurements to prevent charging. All data were collected using a 72-W focused X-ray beam with a spot size of 50 μm at a base pressure of 5 × 10–7 millibar or lower. Survey scans were obtained with a pass energy of 200 eV and a step size of 1 eV. The work function values from UPS and elemental compositions from XPS were calculated using the Thermo Avantage software package (v5.9925).

Sum-Frequency Generation (SFG) Vibrational Spectroscopy

The SFG samples were prepared using the homopolymer solutions and spin-coating procedure described above. Here, ∼ 4 nm thick polymer films were deposited on graphene/SiO2 (quartz) and SiO2 (control) surfaces. SFG experiments were performed using a home-built SFG spectrometer reported previously. (52−54) SFG signals were collected in a reflection geometry, where colinearly propagating narrowband near-infrared (NIR, λNIR ∼ 803 nm) and broadband mid-infrared (IR, λIR ∼ 3390 nm) beams were temporally and spatially overlapped at the polymer samples at a 60° angle relative to the surface normal. Samples were placed such that the polymer films were facing upright, and the graphene layer buried beneath. Control experiments on graphene-free samples used the same geometry but variable polymer layer thicknesses. Polarizations for both NIR and IR beams were varied with half-waveplates and the radiated SFG signal was polarization resolved using an achromatic half-waveplate/polarizer pair. Letters describing SFG polarization combinations (e.g., SSP) denote the polarization state of the SFG, NIR, and IR fields, respectively. Single frame exposure times of 3 min were used for measurements and were averaged over 3 frames. Background-subtracted SFG spectra were scaled by the response from gold films collected in the PPP combination. Intensity spectra were fit using
ISFG∝|∑qAqωIR−ωq+iΓq+χNR(2)eiϕ|2
(1)
where ISFG is the measured SFG intensity, χNR(2) is the nonresonant background and ϕ is the phase angle. Amplitudes (Aq), resonant frequencies (ωq), and peak widths (Γq) are the parameters for the fit vibrational modes. (52−55) A summary of all fitting parameters is provided in the Supporting Information.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acscentsci.4c00704.

  • Synthetic methods and characterization of the monomers and polymers; ellipsometry data; supporting SFG spectra, fitting results, details on orientational analysis; supplementary KPFM and AFM profiles, DSC curves, and optical images (PDF)

Terms & Conditions

Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Authors
  • Authors
    • James Nicolas Pagaduan - Polymer Science and Engineering Department, University of Massachusetts, Amherst, Massachusetts 01003, United States;  Orcidhttps://orcid.org/0000-0001-6565-3553
    • Nicholas Hight-Huf - Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003, United States;  Orcidhttps://orcid.org/0000-0002-9952-9187
    • Le Zhou - Polymer Science and Engineering Department, University of Massachusetts, Amherst, Massachusetts 01003, United States
    • Nicholas Dix - Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003, United States
    • Uvinduni I. Premadasa - Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States;  Orcidhttps://orcid.org/0000-0003-0289-2965
    • Benjamin Doughty - Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States;  Orcidhttps://orcid.org/0000-0001-6429-9329
    • Thomas P. Russell - Polymer Science and Engineering Department, University of Massachusetts, Amherst, Massachusetts 01003, United States;  Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States;  Orcidhttps://orcid.org/0000-0001-6384-5826
    • Ashwin Ramasubramaniam - Department of Mechanical and Industrial Engineering and Materials Science Graduate Program, University of Massachusetts, Amherst, Massachusetts 01003, United States;  Orcidhttps://orcid.org/0000-0001-6595-7442
    • Michael Barnes - Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003, United States
  • Author Contributions

    All the authors contributed to the preparation of this manuscript. J.N.P. performed conceptualization, sample preparation, UPS and XPS measurements, visualization, project administration, and wrote the original draft. L.Z. synthesized and characterized the monomers and polymers. N.H., N.D., and M.B. performed KPFM measurements. U.I.P and B.D. conducted SFG spectroscopy measurements and analyses. T.E. and R.K. supervised the project and acquired funding.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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We gratefully acknowledge support from the National Science Foundation (NSF-BSF-1808011) and NSF-CHE-2203578. R.K. expresses gratitude for startup funding from UMass Amherst and a 3M Non-Tenured Faculty Award. J.N.P. thanks Dr. H. Greg Lin for assistance with UPS and XPS measurements and Dr. Zhefei Yang for assistance with ellipsometry measurements. This work was performed in part at the Harvard University Center for Nanoscale Systems (CNS), a member of the National Nanotechnology Coordinated Infrastructure Network (NNCI), which is supported by the National Science Foundation under NSF award 1541959. SFG work by U.I.P. and B.D. was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division; this work was produced by UT-Battelle LLC under Contract No. AC05-00OR22725 with the U.S. Department of Energy. T.P.R. was supported by the Air Force Office of Scientific Research under contract FA9550-21-1-0388.

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    Journal of the American Chemical Society (2006), 128 (45), 14422-14423CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Counterion exchange in cationic conjugated polyelectrolytes provides for a straightforward method to modulate the performance of these materials as the electron transport (injection) layer (ETL) in org. light emitting diodes. The bromide counterions of [(9,9-bis(6'-N,N,N-trimethylammonium)-hexyl)fluorene]bromide (PF-Br) can be easily replaced with trifluoromethylsulfonate (PF-CF3SO3), tetrakis(imidazolyl)borate (PF-BIm4) or tetrakis(3,5-trifluoromethylphenyl)borate (PF-BArF4) by a procedure that involves pptn. and washing. The performance of LEDs using MEH-PPV as the emissive layer, Al as the cathode, and the conjugated polyelectrolytes as the ETL varies in the order: PF-BIm4 > PF-CF3SO3 > PF-Br > PF-BArF4. In the case of PF-BIm4, the luminous efficiencies of the devices are similar to those of devices using Ba as the cathode. Thus, by properly choosing the counterion one can use higher work function metals that are more stable than lower work function metals without a substantial barrier to electron injection.
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  15. 15
    Zhang, Y.-Q.; Lin, H.-A.; Pan, Q.-C.; Qian, S.-H.; Zhang, S.-H.; Qiu, G.; Luo, S.-C.; Yu, H.; Zhu, B. Tunable Protein/Cell Binding and Interaction with Neurite Outgrowth of Low-Impedance Zwitterionic PEDOTs. ACS Appl. Mater. Interfaces 2020, 12 (10), 12362– 12372,  DOI: 10.1021/acsami.9b23025
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    15
    Tunable Protein/Cell Binding and Interaction with Neurite Outgrowth of Low-Impedance Zwitterionic PEDOTs
    Zhang, Ya-Qiong; Lin, Hsing-An; Pan, Qi-Chao; Qian, Si-Hao; Zhang, Shu-Hua; Qiu, Gao; Luo, Shyh-Chyang; Yu, Hsiao-hua; Zhu, Bo
    ACS Applied Materials & Interfaces (2020), 12 (10), 12362-12372CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Zwitterionic poly(3,4-ethylenedioxythiophene) (PEDOT) is an effective electronic material for bioelectronics because it exhibits efficient elec. trade-off and diminishes immune response. To promote the use of zwitterionic PEDOTs in bioelectronic devices, esp. for cell alignment control and close electrocoupling, features such as tunable interaction of PEDOTs with proteins/cells and spatially modulating cell behavior are required. However, there is a lack of reliable methods to assemble zwitterionic EDOTs with other functionalized EDOT materials, having different polarities and oxidn. potentials, to prep. PEDOTs with the aforementioned surface properties. In this study, the authors have developed a surfactant-assisted electropolymn. to assemble phosphorylcholine (PC)-functionalized EDOT with other functionalized EDOTs. By adjusting compns., the interaction of PEDOT copolymers with proteins/cells can be finely tuned; the compn. adjustment has an ignorable influence on the impedance of the copolymers. The authors also demonstrate that the cell-repulsive force generated from PC can spatially guide the neurite outgrowth to form a neuron network at single-cell resoln. and greatly enhance the neurite outgrowth by 179%, which is significantly more distinctive than the reported topog. effect. The authors expect that the derived tunable protein/cell interaction and the PC-induced repulsive guidance for the neurite outgrowth can make low-impedance zwitterionic PEDOTs more useful in bioelectronics.
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  16. 16
    Liu, Y.; Duzhko, V. V.; Page, Z. A.; Emrick, T.; Russell, T. P. Conjugated Polymer Zwitterions: Efficient Interlayer Materials in Organic Electronics. Acc. Chem. Res. 2016, 49 (11), 2478– 2488,  DOI: 10.1021/acs.accounts.6b00402
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    16
    Conjugated Polymer Zwitterions: Efficient Interlayer Materials in Organic Electronics
    Liu, Yao; Duzhko, Volodimyr V.; Page, Zachariah A.; Emrick, Todd; Russell, Thomas P.
    Accounts of Chemical Research (2016), 49 (11), 2478-2488CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    A review. Conjugated polymer zwitterions (CPZs) are neutral, hydrophilic, polymer semiconductors. The pendent zwitterions, viewed as side chain dipoles, impart soly. in polar solvents for soln. processing, and open opportunities as interfacial components of optoelectronic devices, for example, between metal electrodes and org. semiconductor active layers. Such interlayers are crucial for defining the performance of org. electronic devices, e.g., field-effect transistors (OFETs), light-emitting diodes (OLEDs), and photovoltaics (OPVs), all of which consist of multilayer structures. The interlayers reduce the Schottky barrier height and thus improve charge injection in OFETs and OLEDs. In OPVs, the interlayers serve to increase the built-in elec. p.d. (Vbi) across the active layer, ensuring efficient extn. of photogenerated charge carriers. In general, polar and even charged electronically active polymers have gained recognition for their ability to modify metal/semiconductor interfaces to the benefit of org. electronics. While conjugated polyelectrolytes (CPEs) as interlayer materials are well-documented, open questions remain about the role of mobile counterions in CPE-contg. devices. CPZs possess the processing advantages of CPEs, but as neutral mols. lack any potential complications assocd. with counterions. The electronic implications of CPZs on metal electrodes stem from the orientation of the zwitterion dipole moment in close proximity to the metal surface, and the resultant surface-induced polarization. This generates an interfacial dipole (Δ) at the CPZ/metal interface, altering the work function of the electrode, as confirmed by UPS, and improving device performance. An ideal cathode interlayer would reduce electrode work function, have orthogonal processability to the active layer, exhibit good film forming properties (i.e., wettability/uniformity), prevent exciton quenching, possess optimal electron affinity that neither limits the work function redn. nor impedes the charge extn., transport electrons selectively, and exhibit long-term stability. The authors' recent discoveries show that CPZs achieve many of these attributes, and are poised for further expansion and development in the interfacial science of org. electronics. This Account reviews a recent collaboration that began with the synthesis of CPZs and a study of their structural and electronic properties on metals, then extended to their application as interlayer materials for OPVs. CPZ structure-property relations based on several material platforms, ranging from homopolymers to copolymers, and from materials with intrinsic p-type conjugated backbones to those with intrinsic n-type conjugated backbones are discussed. Key components of such interlayers, including (i) the origin of work function redn. of CPZ interlayers on metals; (ii) the role of the frontier MO energy levels and their trade-offs in optimizing electronic and device properties; and (iii) the role of polymer cond. type and the magnitude of charge carrier mobility are discussed. The authors' motivation is to present the authors' prior use and current understanding of CPZs as interlayer materials in org. electronics, and describe outstanding issues and future potential directions.
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  17. 17
    Liu, Y.; Sheri, M.; Cole, M. D.; Emrick, T.; Russell, T. P. Combining Fullerenes and Zwitterions in Non-Conjugated Polymer Interlayers to Raise Solar Cell Efficiency. Angew. Chem., Int. Ed. 2018, 57 (31), 9675– 9678,  DOI: 10.1002/anie.201803748
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    17
    Combining Fullerenes and Zwitterions in Non-Conjugated Polymer Interlayers to Raise Solar Cell Efficiency
    Liu, Yao; Sheri, Madhu; Cole, Marcus D.; Emrick, Todd; Russell, Thomas P.
    Angewandte Chemie, International Edition (2018), 57 (31), 9675-9678CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Polymer zwitterions were synthesized by nucleophilic ring-opening of 3,3'-(but-2-ene-1,4-diyl)bis(1,2-oxathiolane 2,2-dioxide) (a bis-sultone) with functional perylene diimide (PDI) or fullerene monomers. Integration of these polymers into solar cell devices as cathode interlayers boosted efficiencies of fullerene-based org. photovoltaics (OPVs) from 2.75 % to 10.74 %, and of non-fullerene-based OPVs from 4.25 % to 10.10 %, demonstrating the versatility of these interlayer materials in OPVs. The fullerene-contg. polymer zwitterion (C60-PZ) showed a higher interfacial dipole (Δ) value and electron mobility than its PDI counterpart (PDI-PZ), affording solar cells with high efficiency. The power of PDI-PZ and C60-PZ to improve electron injection and extn. processes when positioned between metal electrodes and org. semiconductors highlights their promise to overcome energy barriers at the hard-soft materials interface of org. electronics.
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  18. 18
    Alon, H.; Stern, C.; Kirshner, M.; Sinai, O.; Wasserman, M.; Selhorst, R.; Gasper, R.; Ramasubramaniam, A.; Emrick, T.; Naveh, D. Lithographically Patterned Functional Polymer-Graphene Hybrids for Nanoscale Electronics. ACS Nano 2018, 12 (2), 1928– 1933,  DOI: 10.1021/acsnano.7b08844
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    18
    Lithographically patterned functional polymer-graphene hybrids for nanoscale electronics
    Alon, Hadas; Stern, Chen; Kirshner, Moshe; Sinai, Ofer; Wasserman, Michal; Selhorst, Ryan; Gasper, Raymond; Ramasubramaniam, Ashwin; Emrick, Todd; Naveh, Doron
    ACS Nano (2018), 12 (2), 1928-1933CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Two-dimensional (2D) materials are believed to hold significant promise in nanoscale optoelectronics. While significant progress has been made in this field over the past decade, the ability to control charge carrier d. with high spatial precision remains an outstanding challenge in 2D devices. The authors present an approach that simultaneously addresses the dual issues of charge-carrier doping and spatial precision based on a functional lithog. resist that employs methacrylate polymers contg. zwitterionic sulfobetaine pendent groups for noncovalent surface doping of 2D materials. They demonstrate scalable approaches for patterning these polymer films via electron-beam lithog., achieving precise spatial control over carrier doping for fabrication of high-quality, all-2D, lateral p-n junctions in graphene. Their approach preserves all of the desirable structural and electronic properties of graphene while exclusively modifying its surface potential. The functional polymer resist platform and concept offers a facile route toward lithog. doping of graphene- and other 2D material-based optoelectronic devices.
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  19. 19
    Lee, H.; Puodziukynaite, E.; Zhang, Y.; Stephenson, J. C.; Richter, L. J.; Fischer, D. A.; DeLongchamp, D. M.; Emrick, T.; Briseno, A. L. Poly(Sulfobetaine Methacrylate)s as Electrode Modifiers for Inverted Organic Electronics. J. Am. Chem. Soc. 2015, 137 (1), 540– 549,  DOI: 10.1021/ja512148d
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    19
    Poly(sulfobetaine methacrylate)s as Electrode Modifiers for Inverted Organic Electronics
    Lee, Hyunbok; Puodziukynaite, Egle; Zhang, Yue; Stephenson, John C.; Richter, Lee J.; Fischer, Daniel A.; DeLongchamp, Dean M.; Emrick, Todd; Briseno, Alejandro L.
    Journal of the American Chemical Society (2015), 137 (1), 540-549CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The authors demonstrate the use of poly(sulfobetaine methacrylate) (PSBMA), and its pyrene-contg. copolymer, as soln.-processable work function reducers for inverted org. electronic devices. A notable feature of PSBMA is its orthogonal soly. relative to solvents typically employed in the processing of org. semiconductors. A strong permanent dipole moment on the sulfobetaine moiety was calcd. by d. functional theory. PSBMA interlayers reduced the work function of metals, graphene, and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) by over 1 eV, and an ultrathin interlayer of PSBMA reduced the electron injection barrier between indium tin oxide (ITO) and C70 by 0.67 eV. As a result, the performance of org. photovoltaic devices with PSBMA interlayers is significantly improved, and enhanced electron injection is demonstrated in electron-only devices with ITO, PEDOT:PSS, and graphene electrodes. This work makes available a new class of dipole-rich, counterion-free, pH insensitive polymer interlayers with demonstrated effectiveness in inverted devices.
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  20. 20
    Pagaduan, J. N.; Hight-Huf, N.; Datar, A.; Nagar, Y.; Barnes, M.; Naveh, D.; Ramasubramaniam, A.; Katsumata, R.; Emrick, T. Electronic Tuning of Monolayer Graphene with Polymeric “Zwitterists.. ACS Nano 2021, 15 (2), 2762– 2770,  DOI: 10.1021/acsnano.0c08624
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    20
    Electronic tuning of monolayer graphene with polymeric zwitterists
    Pagaduan, James Nicolas; Hight-Huf, Nicholas; Datar, Avdhoot; Nagar, Yehiel; Barnes, Michael; Naveh, Doron; Ramasubramaniam, Ashwin; Katsumata, Reika; Emrick, Todd
    ACS Nano (2021), 15 (2), 2762-2770CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Work function engineering of two-dimensional (2D) materials by application of polymer coatings represents a research thrust that promises to enhance the performance of electronic devices. While polymer zwitterions have been demonstrated to significantly modify the work function of both metal electrodes and 2D materials due to their dipole-rich structure, the impact of zwitterion chem. structure on work function modulation is not well understood. To address this knowledge gap, we synthesized a series of sulfobetaine-based zwitterionic random copolymers with variable substituents and used them in lithog. patterning for the prepn. of neg.-tone resists (i.e., zwitterists) on monolayer graphene. UPS indicated a significant work function redn., as high as 1.5 eV, induced by all polymer zwitterions when applied as ultrathin films (<10 nm) on monolayer graphene. Of the polymers studied, the piperidinyl-substituted version, produced the largest dipole normal to the graphene sheet, thereby inducing the max. work function redn. D. functional theory calcns. probed the influence of zwitterion compn. on dipole orientation, while lithog. patterning allowed for evaluation of surface potential contrast via Kelvin probe force microscopy. Overall, this polymer zwitterist design holds promise for fine-tuning 2D materials electronics with spatial control based on the chem. of the polymer coating and the dimensions of the lithog. patterning.
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  21. 21
    Hight-Huf, N.; Nagar, Y.; Levi, A.; Pagaduan, J. N.; Datar, A.; Katsumata, R.; Emrick, T.; Ramasubramaniam, A.; Naveh, D.; Barnes, M. D. Polarization-Driven Asymmetric Electronic Response of Monolayer Graphene to Polymer Zwitterions Probed from Both Sides. ACS Appl. Mater. Interfaces 2021, 13, 47945,  DOI: 10.1021/acsami.1c13505
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    21
    Polarization-Driven Asymmetric Electronic Response of Monolayer Graphene to Polymer Zwitterions Probed from Both Sides
    Hight-Huf, Nicholas; Nagar, Yehiel; Levi, Adi; Pagaduan, James Nicolas; Datar, Avdhoot; Katsumata, Reika; Emrick, Todd; Ramasubramaniam, Ashwin; Naveh, Doron; Barnes, Michael D.
    ACS Applied Materials & Interfaces (2021), 13 (40), 47945-47953CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    We investigated the nature of graphene surface doping by zwitterionic polymers and the implications of weak in-plane and strong through-plane screening using a novel sample geometry that allows direct access to either the graphene or the polymer side of a graphene/polymer interface. Using both Kelvin probe and electrostatic force microscopies, we obsd. a significant upshift in the Fermi level in graphene of ~ 260 meV that was dominated by a change in polarizability rather than pure charge transfer with the org. overlayer. This phys. picture is supported by d. functional theory (DFT) calcns., which describe a redistribution of charge in graphene in response to the dipoles of the adsorbed zwitterionic moieties, analogous to a local DC Stark effect. Strong metallic-like screening of the adsorbed dipoles was obsd. by employing an inverted geometry, an effect identified by DFT to arise from a strongly asym. redistribution of charge confined to the side of graphene proximal to the zwitterion dipoles. Transport measurements confirm n-type doping with no significant impact on carrier mobility, thus demonstrating a route to desirable electronic properties in devices that combine graphene with lithog. patterned polymers.
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  22. 22
    Zhou, L.; Triozzi, A.; Figueiredo, M.; Emrick, T. Fluorinated Polymer Zwitterions: Choline Phosphates and Phosphorylcholines. ACS Macro Lett. 2021, 10 (10), 1204– 1209,  DOI: 10.1021/acsmacrolett.1c00451
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    22
    Fluorinated Polymer Zwitterions: Choline Phosphates and Phosphorylcholines
    Zhou, Le; Triozzi, Alexandria; Figueiredo, Marxa; Emrick, Todd
    ACS Macro Letters (2021), 10 (10), 1204-1209CODEN: AMLCCD; ISSN:2161-1653. (American Chemical Society)
    Among zwitterionic structures, the choline phosphate (CP) group is uniquely attractive for its ability to access novel chem. compns. that embed functional groups directly into the zwitterionic moiety. This paper describes the attachment of fluorinated alkyl groups to CP moieties, yielding zwitterionic monomers 1 and 2 that proved amenable to controlled free radical polymn. and the prodn. of a new set of CP-contg. fluorinated polymers and copolymers with phosphorylcholine (PC) zwitterions. This combination of fluorinated hydrocarbons and zwitterions affords novel, water-sol. polymeric amphiphiles that we have examd. at fluid interfaces, as coatings, in cell culture, and in magnetic resonance imaging.
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  23. 23
    Zhou, L.; Yang, Z.; Pagaduan, J. N.; Emrick, T. Fluorinated Zwitterionic Polymers as Dynamic Surface Coatings. Polym. Chem. 2022, 14 (1), 32– 36,  DOI: 10.1039/D2PY01197B
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    There is no corresponding record for this reference.
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    Borkar, S.; Jankova, K.; Siesler, H. W.; Hvilsted, S. New Highly Fluorinated Styrene-Based Materials with Low Surface Energy Prepared by ATRP. Macromolecules 2004, 37 (3), 788– 794,  DOI: 10.1021/ma034952b
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    New highly fluorinated styrene-based materials with low surface energy prepared by ATRP
    Borkar, Sachin; Jankova, Katja; Siesler, Heinz W.; Hvilsted, Soren
    Macromolecules (2004), 37 (3), 788-794CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
    2,3,5,6-Tetrafluoro-4-(2,2,3,3,3-pentafluoropropoxy)styrene [TF(F5)S] and 2,3,5,6-tetrafluoro-4-(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyloxy)styrene [TF(F15)S] are prepd. by nucleophilic substitution of 2,3,4,5,6-pentafluorostyrene. The neat monomers are subjected to atom transfer radical polymn. (ATRP) at 110°C to high conversions in relatively short times (10-120 min); TF(F5)S is addnl. polymd. at 70 and 90°C. Block copolymers with styrene are prepd. by the macroinitiator approach. All polymers, in the no.-av. mol. wt. range from 6000 to 35,000, have polydispersity indexes between 1.08 and 1.37. The homopolymers show glass transitions from 16 to 62°C depending on mol. wt., whereas the block copolymers exhibit phase sepn. mirrored in two glass temps., which could be obsd. when the smallest block constitutes more than 10 mol%. The fluorinated side chains of P(TF(F5)S) and P(TF(F15)S) enrich the surface of thin films, which results in an advancing water contact angle of 117° and 122°, resp. Both XPS analyses and contact angle measurements strongly imply that the fluorinated parts of the block copolymers migrate to the surface and create low surface energy films.
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  25. 25
    Ming, W.; Tian, M.; van de Grampel, R. D.; Melis, F.; Jia, X.; Loos, J.; van der Linde, R. Low Surface Energy Polymeric Films from Solventless Liquid Oligoesters and Partially Fluorinated Isocyanates. Macromolecules 2002, 35 (18), 6920– 6929,  DOI: 10.1021/ma020650i
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    Low Surface Energy Polymeric Films from Solventless Liquid Oligoesters and Partially Fluorinated Isocyanates
    Ming, W.; Tian, M.; van de Grampel, R. D.; Melis, F.; Jia, X.; Loos, J.; van der Linde, R.
    Macromolecules (2002), 35 (18), 6920-6929CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
    Partially fluorinated isocyanates were synthesized from hexamethylene diisocyanate (HDI) and an HDI trimer (Desmodur N 3300). The perfluoroalkyl group (Rf) was C6F13 or C8F17, and the ratio between Rf and the isocyanate group (NCO) was 1/99 in the case of HDI or 1/49 in the case of N 3300. Polymeric films with surface energies as low as 10 mN/m were obtained from mixts. of these partially fluorinated isocyanates and a previously reported hydroxyl-end-capped solventless liq. oligoester. Contact angles of water and hexadecane reached 120° and 80°, resp., when less than 1 wt.% of fluorine was present in the films. The surface enrichment of fluorine-contg. species was confirmed by XPS investigations. At a fluorine concn. of 0.5-1.0 wt.%, the surface F/C at. ratio (at a 15° takeoff angle) was greater than 1; the surface enrichment factor of fluorine was up to 600. The topol. structures of the polymeric films were recorded by an at. force microscope under tapping mode. While the height images indicated that the surface was smooth at the nanometer scale in a 1 μm × 1 μm area, the phase images revealed that fluorine-enriched domains were present at the surface. As the fluorine concn. increased, the fluorine-enriched domains grew from tiny spots (2-3 nm) to larger round domains (15-25 nm in diam.). The low surface energies of the films could be ascribed to the strong surface segregation of fluorinated species.
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    Hight-Huf, N.; Pagaduan, J. N.; Katsumata, R.; Emrick, T.; Barnes, M. D. Stabilization of Three-Particle Excitations in Monolayer MoS2 by Fluorinated Methacrylate Polymers. J. Phys. Chem. Lett. 2022, 13 (21), 4794– 4799,  DOI: 10.1021/acs.jpclett.2c01150
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    There is no corresponding record for this reference.
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    Saha, S.; Samanta, P.; Murmu, N. C.; Banerjee, A.; Ganesh, R. S.; Inokawa, H.; Kuila, T. Modified Electrochemical Charge Storage Properties of H-BN/rGO Superlattice through the Transition from n to p Type Semiconductor by Fluorine Doping. Chemical Engineering Journal 2018, 339, 334– 345,  DOI: 10.1016/j.cej.2018.01.141
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    Melitz, W.; Shen, J.; Kummel, A. C.; Lee, S. Kelvin Probe Force Microscopy and Its Application. Surf. Sci. Rep. 2011, 66 (1), 1– 27,  DOI: 10.1016/j.surfrep.2010.10.001
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    Wang, P.; Selhorst, R.; Emrick, T.; Ramasubramaniam, A.; Barnes, M. D. Bidirectional Electronic Tuning of Single-Layer MoS2 with Conjugated Organochalcogens. J. Phys. Chem. C 2019, 123 (2), 1506– 1511,  DOI: 10.1021/acs.jpcc.8b10826
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    29
    Bidirectional Electronic Tuning of Single-Layer MoS2 with Conjugated Organochalcogens
    Wang, Peijian; Selhorst, Ryan; Emrick, Todd; Ramasubramaniam, Ashwin; Barnes, Michael D.
    Journal of Physical Chemistry C (2019), 123 (2), 1506-1511CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    We report a bidirectional tuning of the electronic properties of single-layer molybdenum disulfide (MoS2) by n-doping with the electron donating tetrathiafulvalene (TTF) and p-doping with the electron accepting bithiazolidinylidene (BT). Using Kelvin probe force microscopy (KPFM), we spatially monitored changes in the work function of monolayer MoS2 on silicon oxide (SiO2) and sapphire (Al2O3). KPFM, in conjunction with spectroscopic characterization, showed MoS2 work function shifts as significant as 1.24 eV for TTF doping, and 0.43 eV for BT doping, when Al2O3 was employed as the underlying substrate. Less dramatic changes were obsd. for MoS2 on SiO2/Si, revealing a significant impact of substrate selection on the electronic properties of this 2D material. High-level computations helped guide expts. on chem. modulation of the electronic properties of this transition-metal dichalcogenide.
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    Wang, X.; Xu, J.-B.; Xie, W.; Du, J. Quantitative Analysis of Graphene Doping by Organic Molecular Charge Transfer. J. Phys. Chem. C 2011, 115 (15), 7596– 7602,  DOI: 10.1021/jp200386z
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    Quantitative Analysis of Graphene Doping by Organic Molecular Charge Transfer
    Wang, Xiaomu; Xu, Jian-Bin; Xie, Weiguang; Du, Jun
    Journal of Physical Chemistry C (2011), 115 (15), 7596-7602CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Both n-type and p-type doped exfoliated graphene sheets are presented by virtue of adsorbing org. mols. Flat org. layers are uniformly grown on graphene sheets by the technique. Meanwhile, the high-mobility attribute of graphene is largely preserved, as adsorption on the graphene surface does not significantly modify the π-bonding networks of graphene. By employing Kelvin probe force microscopy, the 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) mols. obtain electrons from graphene whereas vanadyl-phthalocyanine (VOPc) mols. donate electrons to it. The amt. of charge transfer by F4-TCNQ and VOPc to bilayer graphene on silicon dioxide substrate is ∼0.4 and 0.1 electron/mol. by a tight-binding self-consistent model, resp. The consistent theor. and doping of graphene by org. mol. charge transfer has great implications for future large-scale applications of graphene-based nanoelectronics.
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    Zhou, X.; He, S.; Brown, K. A.; Mendez-Arroyo, J.; Boey, F.; Mirkin, C. A. Locally Altering the Electronic Properties of Graphene by Nanoscopically Doping It with Rhodamine 6G. Nano Lett. 2013, 13 (4), 1616– 1621,  DOI: 10.1021/nl400043q
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    Locally Altering the Electronic Properties of Graphene by Nanoscopically Doping It with Rhodamine 6G
    Zhou, Xiaozhu; He, Shu; Brown, Keith A.; Mendez-Arroyo, Jose; Boey, Freddy; Mirkin, Chad A.
    Nano Letters (2013), 13 (4), 1616-1621CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Rhodamine 6G (R6G), patterned by dip-pen nanolithog. on graphene, can be used to locally n-dope it in a controlled fashion. The authors study the transport and assembly properties of R6G on graphene and show that in general the π-π stacking between the arom. components of R6G and the underlying graphene drives the assembly of these mols. onto the underlying substrate. However, two distinct transport and assembly behaviors, dependent upon the presence or absence of R6G dimers, were identified. In particular, at high concns. of R6G on the tip, dimers are transferred to the substrate and form contiguous and stable lines, while at low concns., the R6G is transferred as monomers and forms patchy, unstable, and relatively ill-defined features. Finally, Kelvin probe force microscopy expts. show that the local electrostatic potential of the graphene changes as function of modification with R6G; this behavior is consistent with local mol. doping, highlighting a path for controlling the electronic properties of graphene with nanoscale resoln.
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    Leckey, R. Ultraviolet Photoelectron Spectroscopy of Solids. In Surface Analysis Methods in Materials Science; O’Connor, D. J., Sexton, B. A., Smart, R. St. C., Eds.; Springer Series in Surface Sciences; Springer: Berlin, Heidelberg, 2003; pp 337– 345.  DOI: 10.1007/978-3-662-05227-3_14 .
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    Lee, W. H.; Suk, J. W.; Lee, J.; Hao, Y.; Park, J.; Yang, J. W.; Ha, H.-W.; Murali, S.; Chou, H.; Akinwande, D.; Kim, K. S.; Ruoff, R. S. Simultaneous Transfer and Doping of CVD-Grown Graphene by Fluoropolymer for Transparent Conductive Films on Plastic. ACS Nano 2012, 6 (2), 1284– 1290,  DOI: 10.1021/nn203998j
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    Simultaneous Transfer and Doping of CVD-Grown Graphene by Fluoropolymer for Transparent Conductive Films on Plastic
    Lee, Wi Hyoung; Suk, Ji Won; Lee, Jongho; Hao, Yufeng; Park, Jaesung; Yang, Jae Won; Ha, Hyung-Wook; Murali, Shanthi; Chou, Harry; Akinwande, Deji; Kim, Kwang S.; Ruoff, Rodney S.
    ACS Nano (2012), 6 (2), 1284-1290CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Chem. doping can decrease sheet resistance of graphene while maintaining its high transparency. The authors report a new method to simultaneously transfer and dope CVD grown graphene onto a target substrate using a fluoropolymer as both the supporting and doping layer. Solvent was used to remove a significant fraction of the supporting fluoropolymer, but residual polymer remained that doped the graphene significantly. This contrasts with a more widely used supporting layer, polymethylmethacrylate, which does not induce significant doping during transfer. The fluoropolymer doping mechanism can be explained by the rearrangement of fluorine atoms on the graphene basal plane caused by either thermal annealing or soaking in solvent, which induces ordered dipole moments near the graphene surface. This simultaneous transfer and doping of the graphene with a fluoropolymer increases the carrier d. significantly, and the resulting monolayer graphene film exhibits a sheet resistance of ∼320 Ω/sq. Finally, the method presented here was used to fabricate flexible and a transparent graphene electrode on a plastic substrate.
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    Liu, F.; Page, Z. A.; Duzhko, V. V.; Russell, T. P.; Emrick, T. Conjugated Polymeric Zwitterions as Efficient Interlayers in Organic Solar Cells. Adv. Mater. 2013, 25 (47), 6868– 6873,  DOI: 10.1002/adma.201302477
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    Conjugated polymeric zwitterions as efficient interlayers in organic solar cells
    Liu, Feng; Page, Zachariah A.; Duzhko, Volodimyr V.; Russell, Thomas P.; Emrick, Todd
    Advanced Materials (Weinheim, Germany) (2013), 25 (47), 6868-6873CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Inserting conjugated polymeric zwitterions between the metal cathode and active PTB7/PC71BM layer resulted in a PCE increase of >500% (0.92% to 5.78% for Ag-cathode devices, and 68% (4.39% to 7.36%) for Al-contg. devices.
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    Crispin, X.; Geskin, V.; Crispin, A.; Cornil, J.; Lazzaroni, R.; Salaneck, W. R.; Brédas, J.-L. Characterization of the Interface Dipole at Organic/ Metal Interfaces. J. Am. Chem. Soc. 2002, 124 (27), 8131– 8141,  DOI: 10.1021/ja025673r
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    Characterization of the Interface Dipole at Organic/ Metal Interfaces
    Crispin, Xavier; Geskin, Victor; Crispin, Annica; Cornil, Jerome; Lazzaroni, Roberto; Salaneck, William R.; Bredas, Jean-Luc
    Journal of the American Chemical Society (2002), 124 (27), 8131-8141CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    In orgs.-based (opto)electronic devices, the interface dipoles formed at the org./metal interfaces play a key role in detg. the barrier for charge (hole or electron) injection between the metal electrodes and the active org. layers. The origin of this dipole is rationalized here from the results of a joint exptl. and theor. study based on the interaction between acrylonitrile, a π-conjugated mol., and transition metal surfaces (Cu, Ni, and Fe). The adsorption of acrylonitrile on these surfaces was studied exptl. by photoelectron spectroscopies, while quantum mech. methods based on d. functional theory were used to study the systems theor. Apparently the interface dipole formed at an org./metal interface can be divided into two contributions: (i) the 1st corresponds to the chem. dipole induced by a partial charge transfer between the org. layers and the metal upon chemisorption of the org. mols. on the metal surface, and (ii) the 2nd relates to the change in metal surface dipole because of the modification of the metal electron d. tail that is induced by the presence of the adsorbed org. mols. The authors' anal. shows that the charge injection barrier in devices can be tuned by modulating various parameters: the chem. potential of the bare metal (given by its work function), the metal surface dipole, and the ionization potential and electron affinity of the org. layer.
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    Terrones, H.; Lv, R.; Terrones, M.; Dresselhaus, M. S. The Role of Defects and Doping in 2D Graphene Sheets and 1D Nanoribbons. Rep. Prog. Phys. 2012, 75 (6), 062501,  DOI: 10.1088/0034-4885/75/6/062501
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    The role of defects and doping in 2D graphene sheets and 1D nanoribbons
    Terrones, Humberto; Lv, Ruitao; Terrones, Mauricio; Dresselhaus, Mildred S.
    Reports on Progress in Physics (2012), 75 (6), 062501/1-062501/30CODEN: RPPHAG; ISSN:0034-4885. (Institute of Physics Publishing)
    A review. Defects are usually seen as imperfections in materials that could significantly degrade their performance. However, at the nanoscale, defects could be extremely useful since they could be exploited to generate novel, innovative and useful materials and devices. Graphene and graphene nanoribbons are no exception. This review therefore tries to categorize defects, emphasize their importance, introduce the common routes to study and identify them and to propose new ways to construct novel devices based on 'defective' graphene-like materials. In particular, we will discuss defects in graphene-like systems including (a) structural (sp2-like) defects, (b) topol. (sp2-like) defects, (c) doping or functionalization (sp2- and sp3-like) defects and (d) vacancies/edge type defects (non-sp2-like). It will be demonstrated that defects play a key role in graphene physicochem. properties and could even be crit. to generate biocompatible materials. There are numerous challenges in this emerging field, and we intend to provide a stimulating account which could trigger new science and technol. developments based on defective graphene-like materials that could be introduced into other at. layered materials, such as BN, MoS2 and WS2, not discussed in this review.
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    McDonnell, S.; Addou, R.; Buie, C.; Wallace, R. M.; Hinkle, C. L. Defect-Dominated Doping and Contact Resistance in MoS2. ACS Nano 2014, 8 (3), 2880– 2888,  DOI: 10.1021/nn500044q
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    Defect-Dominated Doping and Contact Resistance in MoS2
    McDonnell, Stephen; Addou, Rafik; Buie, Creighton; Wallace, Robert M.; Hinkle, Christopher L.
    ACS Nano (2014), 8 (3), 2880-2888CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Achieving low resistance contacts is vital for the realization of nanoelectronic devices based on transition metal dichalcogenides. Intrinsic defects in MoS2 dominate the metal/MoS2 contact resistance and provide a low Schottky barrier independent of metal contact work function. Also, MoS2 can exhibit both n-type and p-type conduction at different points on a same sample. The authors identify these regions independently by complementary characterization techniques and show how the Fermi level can shift by 1 eV over tens of nanometers in spatial resoln. These variations in doping are defect-chem.-related and are independent of contact metal. This raises questions on previous reports of metal-induced doping of MoS2 since the same metal in contact with MoS2 can exhibit both n- and p-type behavior. These results may provide a potential route for achieving low electron and hole Schottky barrier contacts with a single metal deposition.
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    Chowdhury, A. U.; Chang, D.; Xu, Y.; Hong, K.; Sumpter, B. G.; Carrillo, J.-M. Y.; Doughty, B. Mapping the Interfacial Chemistry and Structure of Partially Fluorinated Bottlebrush Polymers and Their Linear Analogues. Langmuir 2021, 37 (1), 211– 218,  DOI: 10.1021/acs.langmuir.0c02786
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    Mapping the Interfacial Chemistry and Structure of Partially Fluorinated Bottlebrush Polymers and Their Linear Analogues
    Chowdhury, Azhad U.; Chang, Dongsook; Xu, Yuewen; Hong, Kunlun; Sumpter, Bobby G.; Carrillo, Jan-Michael Y.; Doughty, Benjamin
    Langmuir (2021), 37 (1), 211-218CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Polymer interfaces are key to a range of applications including membranes for chem. sepns., hydrophobic coatings, and passivating layers for antifouling. While important, challenges remain in probing the interfacial monolayer where the mol. ordering and orientation can change depending on the chem. makeup or processing conditions. In this work, we leverage surface specific vibrational sum frequency generation (SFG) and the assocd. dependence on mol. symmetry to elucidate the ordering and orientations of key functional groups for poly(2,2,2-trifluoroethyl methacrylate) bottlebrush polymers and their linear polymer analogs. These measurements were framed by atomistic mol. dynamic simulations to provide a complementary phys. picture of the gas-polymer interface. Simulations and SFG measurements show that methacrylate backbones are buried beneath a layer of trifluoroethyl contg. side groups that result in structurally similar interfaces regardless of the polymer mol. wt. or architecture. The av. orientational angles of the trifluoroethyl contg. side groups differ depending on polymer linear and bottlebrush architectures, suggesting that the surface groups can reorient via available rotational degrees of freedom. Results show that the surfaces of the bottlebrush and linear polymer samples do not strongly depend on mol. wt. or architecture. As such, one cannot rely on increasing the mol. wt. or altering the architecture to tune surface properties. This insight into the polymer interfacial structure is expected to advance the design of new material interfaces with tailored chem./functional properties.
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    Tateishi, Y.; Kai, N.; Noguchi, H.; Uosaki, K.; Nagamura, T.; Tanaka, K. Local Conformation of Poly(Methyl Methacrylate) at Nitrogen and Water Interfaces. Polym. Chem. 2010, 1 (3), 303– 311,  DOI: 10.1039/B9PY00227H
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    Local conformation of poly(methyl methacrylate) at nitrogen and water interfaces
    Tateishi, Yohei; Kai, Naoki; Noguchi, Hidenori; Uosaki, Kohei; Nagamura, Toshihiko; Tanaka, Keiji
    Polymer Chemistry (2010), 1 (3), 303-311CODEN: PCOHC2; ISSN:1759-9962. (Royal Society of Chemistry)
    The local conformation of poly(Me methacrylate) (PMMA) chains at the nitrogen (N2) and water interfaces was studied by IR-visible sum-frequency generation (SFG) spectroscopy. Although SFG spectra in the C-H region for PMMA at the N2 interface have been hitherto reported, the peak assignments are not in accord with one another. Thus, we first made accurate assignments of SFG peaks using films, which had been well annealed at a temp. above the glass transition temp. for a long time, of three different deuterated PMMAs as well as normal protonated PMMA. At the N2 interface, hydrophobic functional groups such as a Me, ester Me and methylene groups were present. While the a Me group was oriented along the direction parallel to the interface, ester Me and methylene groups were oriented normal to the interface. Quant. discussion concerning the orientation of the functional groups of PMMA at the N2 interface was aided by a model calcn. Once the PMMA film contacted water, the carbonyl groups of the PMMA side chains were oriented to the water phase to form hydrogen bonds with water mols., resulting in the migration of ester Me into the internal region of the film. Concurrently, the methylene groups became randomly oriented at the water interface and/or in part migrated into the internal region. Interestingly, the Me groups still existed at the water interface oriented along the parallel direction. The outermost region of PMMA in water can consist of hydrophilic and hydrophobic domains with sub-nanometer scale. Water mols. H-bond to themselves near the hydrophobic domains, leading to the formation of an ice-like structure of water mols. However, water mols. adjacent to the hydrophilic domains H-bond with carbonyl groups.
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    Cimatu, K. L. A.; Ambagaspitiya, T. D.; Premadasa, U. I.; Adhikari, N. M.; Kruse, A.; Robertson, E.; Guan, S.; Rong, L.; Advincula, R.; Bythell, B. J. Polymer-Solvent Interaction and Conformational Changes at a Molecular Level: Implication to Solvent-Assisted Deformation and Aggregation at the Polymer Surface. J. Colloid Interface Sci. 2022, 616, 221– 233,  DOI: 10.1016/j.jcis.2022.02.006
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    Polymer-solvent interaction and conformational changes at a molecular level: Implication to solvent-assisted deformation and aggregation at the polymer surface
    Cimatu, Katherine Leslee A.; Ambagaspitiya, Tharushi D.; Premadasa, Uvinduni I.; Adhikari, Narendra M.; Kruse, Adelaide; Robertson, Emily; Guan, Shanshan; Rong, Lihan; Advincula, Rigoberto; Bythell, Benjamin J.
    Journal of Colloid and Interface Science (2022), 616 (), 221-233CODEN: JCISA5; ISSN:0021-9797. (Elsevier B.V.)
    We hypothesize that varying the chem. structure of the monomeric unit in a polymer will affect the surface structure and interfacial mol. group orientations of the polymer film leveraging its response to solvents of different chem. affinities. Poly (2-methoxy Et methacrylate) and poly (2-tertbutoxy Et methacrylate) thin films exposed to either deuterated water (D2O) or deuterated chloroform (CDCl3) were studied by sum frequency generation (SFG) spectroscopy, contact angle goniometry, and at. force microscopy (AFM) at the polymer-solvent interface, supported with mol. simulation studies. SFG spectral anal. of the polymer thin films corroborated mol. re-organization at the surface when exposed to different chem. environments. The AFM height images of the polymer surfaces were homogeneously flat under CDCl3 and showed swollen regions under D2O. Following the removal of D2O, the exposed areas have imprinted, recessed locations and exposure to CDCl3 resulted in the formation of aggregates. The chem. affinity and characteristics of the solvents played a role in conformational change at the polymer surface. It had direct implications to interfacial processes involving adsorption, permeation which eventually leads to swelling, deformation or aggregation, and possibly dissoln.
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    Doughty, B.; Genix, A.-C.; Popov, I.; Li, B.; Zhao, S.; Saito, T.; Lutterman, D. A.; Sacci, R. L.; Sumpter, B. G.; Wojnarowska, Z.; Bocharova, V. Structural Correlations Tailor Conductive Properties in Polymerized Ionic Liquids. Phys. Chem. Chem. Phys. 2019, 21 (27), 14775– 14785,  DOI: 10.1039/C9CP02268F
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    Structural correlations tailor conductive properties in polymerized ionic liquids
    Doughty, Benjamin; Genix, Anne-Caroline; Popov, Ivan; Li, Bingrui; Zhao, Sheng; Saito, Tomonori; Lutterman, Daniel A.; Sacci, Robert L.; Sumpter, Bobby G.; Wojnarowska, Zaneta; Bocharova, Vera
    Physical Chemistry Chemical Physics (2019), 21 (27), 14775-14785CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Polymd. ionic liqs. (PolyILs) are promising materials for applications in electrochem. devices spanning from fuel cells to capacitors and batteries. In principle, PolyILs have a competitive advantage over traditional electrolytes in being single ion conductors and thus enabling a transference no. close to unity. Despite this perceived advantage, surprisingly low room temp. ionic conductivities measured in the lab. raise an important fundamental question: how does the mol. structure mediate cond.. In this work, wide-angle X-ray scattering (WAXS), vibrational sum frequency generation (vSFG), and d. functional theory (DFT) calcns. were used to study the bulk and interfacial structure of PolyILs, while broad band dielec. spectroscopy (BDS) was used to probe corresponding dynamics and conductive properties for a series of the PolyIL samples with tunable chemistries and structures. Our results reveal that the size of the mobile anions has a tremendous impact on chain packing in PolyILs that wasn't addressed previously. Larger mobile ions tend to create a well-packed structure, while smaller ions frustrate chain packing. The magnitude of these changes and level of structural heterogeneity are shown to depend on the chem. functionality and flexibility of studied PolyILs. Furthermore, these exptl. and computational results provide new insight into the correlation between cond. and structure in PolyILs, suggesting that structural heterogeneity helps to reduce the activation energy for ionic cond. in the glassy state.
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    Chen, Z. Investigating Buried Polymer Interfaces Using Sum Frequency Generation Vibrational Spectroscopy. Prog. Polym. Sci. 2010, 35 (11), 1376– 1402,  DOI: 10.1016/j.progpolymsci.2010.07.003
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    Investigating buried polymer interfaces using sum frequency generation vibrational spectroscopy
    Chen, Zhan
    Progress in Polymer Science (2010), 35 (11), 1376-1402CODEN: PRPSB8; ISSN:0079-6700. (Elsevier Ltd.)
    A review. This paper reviews recent progress in the studies of buried polymer interfaces using sum frequency generation (SFG) vibrational spectroscopy. Both buried solid/liq. and solid/solid interfaces involving polymeric materials are discussed. SFG studies of polymer/water interfaces show that different polymers exhibit varied surface restructuring behavior in water, indicating the importance of probing polymer/water interfaces in situ. SFG has also been applied to the investigation of interfaces between polymers and other liqs. It has been found that mol. interactions at such polymer/liq. interfaces dictate interfacial polymer structures. The mol. structures of silane mols., which are widely used as adhesion promoters, have been investigated using SFG at buried polymer/silane and polymer/polymer interfaces, providing mol.-level understanding of polymer adhesion promotion. The mol. structures of polymer/solid interfaces have been examd. using SFG with several different exptl. geometries. These results have provided mol.-level information about polymer friction, adhesion, interfacial chem. reactions, interfacial electronic properties, and the structure of layer-by-layer deposited polymers. Such research has demonstrated that SFG is a powerful tool to probe buried interfaces involving polymeric materials, which are difficult to study by conventional surface sensitive anal. techniques.
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    Wang, J.; Paszti, Z.; Even, M. A.; Chen, Z. Measuring Polymer Surface Ordering Differences in Air and Water by Sum Frequency Generation Vibrational Spectroscopy. J. Am. Chem. Soc. 2002, 124 (24), 7016– 7023,  DOI: 10.1021/ja012387r
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    Measuring Polymer Surface Ordering Differences in Air and Water by Sum Frequency Generation Vibrational Spectroscopy
    Wang, Jie; Paszti, Zoltan; Even, Mark A.; Chen, Zhan
    Journal of the American Chemical Society (2002), 124 (24), 7016-7023CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Mol. structures of poly(Bu methacrylate) (PBMA) at the PBMA/air and PBMA/water interfaces have been studied by sum frequency generation (SFG) vibrational spectroscopy. PBMA surfaces in both air and water are dominated by the Me groups of the ester side chains. The av. orientation and orientation distribution of these Me groups at the PBMA/air and PBMA/water interfaces are different, indicating that surface restructuring occurs when the PBMA sample contacts water. Anal. shows that the orientation distribution of side chain Me groups on the PBMA surface is narrower in water than that in air, indicating that the PBMA surface can be more ordered in water. To our knowledge, this is the first time that quant. comparisons between mol. surface structures of polymers in air and in water have been made. Two assumptions on the orientation distribution function, including a Gaussian distribution and a formula based on the max. entropy approach, are used in the anal. It has been found that the orientation angle distribution function deduced by the Gaussian distribution and the max. entropy distribution are quite similar, showing that the Gaussian distribution is a good approxn. for the angle distribution. The effect of exptl. error on the deduced orientational distribution is also discussed.
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    Lambert, A. G.; Davies, P. B.; Neivandt, D. J. Implementing the Theory of Sum Frequency Generation Vibrational Spectroscopy: A Tutorial Review. Appl. Spectrosc. Rev. 2005, 40 (2), 103– 145,  DOI: 10.1081/ASR-200038326
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    Implementing the theory of sum frequency generation vibrational spectroscopy: a tutorial review
    Lambert, Alex G.; Davies, Paul B.; Neivandt, David J.
    Applied Spectroscopy Reviews (2005), 40 (2), 103-145CODEN: APSRBB; ISSN:0570-4928. (Taylor & Francis, Inc.)
    A review. The interfacial regions between bulk media, although often comprising only a fraction of the material present, are frequently the site of reactions and phenomena that dominate the macroscopic properties of the entire system. Spectroscopic investigations of such interfaces are often hampered by the lack of surface specificity of most available techniques. Sum frequency generation vibrational spectroscopy (SFS) is a non-linear optical technique which provides vibrational spectra of mols. solely at interfaces. The spectra may be analyzed to provide the polar orientation, mol. conformation, and av. tilt angle of the adsorbate to the surface normal. This article is aimed at newcomers to the field of SFS, and via a tutorial approach will present and develop the general sum frequency equations and then demonstrate how the fundamental theory elucidates the important exptl. properties of SFS.
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    Wang, H.-F.; Gan, W.; Lu, R.; Rao, Y.; Wu, B.-H. Quantitative Spectral and Orientational Analysis in Surface Sum Frequency Generation Vibrational Spectroscopy (SFG-VS). Int. Rev. Phys. Chem. 2005, 24 (2), 191– 256,  DOI: 10.1080/01442350500225894
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    Quantitative spectral and orientational analysis in surface sum frequency generation vibrational spectroscopy (SFG-VS)
    Wang, Hong-Fei; Gan, Wei; Lu, Rong; Rao, Yi; Wu, Bao-Hua
    International Reviews in Physical Chemistry (2005), 24 (2), 191-256CODEN: IRPCDL; ISSN:0144-235X. (Taylor & Francis Ltd.)
    A review. Sum frequency generation vibrational spectroscopy (SFG-VS) was proven to be a uniquely effective spectroscopic technique in the study of mol. structure and conformations, as well as the dynamics of mol. interfaces. The ability to apply SFG-VS to complex mol. interfaces was limited by the ability to abstr. quant. information from SFG-VS expts. The authors try to make assessments of the limitations, issues and techniques as well as methodols. in quant. orientational and spectral anal. with SFG-VS. Based on these assessments, the authors also try to summarize recent developments in methodologies on quant. orientational and spectral anal. in SFG-VS, and their applications to detailed anal. of SFG-VS data of various vapor/neat liq. interfaces. A rigorous formulation of the polarization null angle (PNA) method is given for accurate detn. of the orientational parameter D = 〈cos θ 〉/〈cos3 θ〉, and comparison between the PNA method with the commonly used polarization intensity ratio (PIR) method is discussed. The polarization and incident angle dependencies of the SFG-VS intensity are reviewed, in the light of how exptl. arrangements can be optimized to effectively abstr. crucial information from the SFG-VS expts. The values and models of the local field factors in the mol. layers are discussed. To examine the validity and limitations of the bond polarizability deriv. model, the general expressions for mol. hyperpolarizability tensors and their expression with the bond polarizability deriv. model for C3v, C2v and C∞v mol. groups are given in the 2 appendixes. The bond polarizability deriv. model can quant. describe many aspects of the intensities obsd. in the SFG-VS spectrum of the vapor/neat liq. interfaces in different polarizations. Using the polarization anal. in SFG-VS, polarization selection rules or guidelines are developed for assignment of the SFG-VS spectrum. Using the selection rules, SFG-VS spectra of vapor/diol, and vapor/n-normal alc. (n ∼ 1-8) interfaces are assigned, and some of the ambiguity and confusion, as well as their implications in previous IR and Raman assignment, are duly discussed. The ability to assign a SFG-VS spectrum using the polarization selection rules makes SFG-VS not only an effective and useful vibrational spectroscopy technique for interface studies, but also a complementary vibrational spectroscopy method in general condensed phase studies. These developments will put quant. orientational and spectral anal. in SFG-VS on a more solid foundation. The formulations, concepts and issues discussed in this review are expected to find broad applications for studies on mol. interfaces in the future.
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    Selective n-type doping of graphene is developed by utilizing patternable gold nanoparticles functionalized with photoreactive cinnamate moieties. The gold nanoparticles can be regularly patterned on the graphene by UV-induced crosslinking of cinnamate, which provides a convenient method to control the optical and elec. properties of graphene site-specifically. The strong n-type doping of graphene covered with the patterned gold nanoparticles was confirmed by Raman, XPS, and electron transport measurements. We believe that our method would find numerous applications in the area of graphene-based optoelectronics including light-emitting devices, solar cells, and optical sensors.
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    Liq./liq. interfaces play a central role in scientific fields ranging from nanomaterial synthesis and soft matter electronics to nuclear waste remediation and chem. sepns. This diversity of functions arises from an interface's ability to respond to changing conditions in its neighboring bulk phases. Understanding what drives this interfacial flexibility can provide novel avenues for designing new functional interfaces. However, limiting this progress is an inadequate understanding of the subtle intermol. and interphase interactions taking place at the mol. level. Here, we use surface-specific vibrational sum frequency generation spectroscopy combined with atomistic mol. dynamics simulations to investigate the self-assembly and structure of model ionic oligomers consisting of an oligodimethylsiloxane (ODMS) tail covalently attached to a pos. charged Me imidazolium (MIM+) head group at buried oil/aq. interfaces. We show how the presence of seemingly innocuous salts can impart dramatic changes to the ODMS tail conformations in the oil phase via specific ion effects and ion-pairing interactions taking place in the aq. phase. These specific ion interactions are shown to drive enhanced amphiphile adsorption, induce morphol. changes, and disrupt emergent hydrogen-bonding structures at the interface. Tuning these interactions allows for independent control over the oligomer structure in the oil phase vs. interfacial population changes and represents key mechanistic insight that is needed to control chem. reactions at liq./liq. interfaces.
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    As fossil fuels remain a major source of energy throughout the world, developing efficient neg. emission technologies, such as direct air capture (DAC), which remove carbon dioxide (CO2) from the air, becomes crit. for mitigating climate change. Although all DAC processes involve CO2 transport from air into a sorbent/solvent, through an air-solid or air-liq. interface, the fundamental roles the interfaces play in DAC remain poorly understood. Herein, we study the interfacial behavior of amino acid (AA) solvents used in DAC through a combination of vibrational sum frequency generation spectroscopy and mol. dynamics simulations. This study revealed that the absorption of atm. CO2 has antagonistic effects on subsequent capture events that are driven by changes in bulk pH and specific ion effects that feedback on surface organization and interactions. Among the three AAs (leucine, valine, and phenylalanine) studied, we identify and sep. behaviors from CO2 loading, chem. changes, variations in pH, and specific ion effects that tune structural and chem. degrees of freedom at the air-aq. interface. The fundamental mechanistic findings described here are anticipated to enable new approaches to DAC based on exploiting interfaces as a tool to address climate change.
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  • Abstract

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    Figure 1

    Figure 1. Top: Synthesis of fluorinated zwitterion-based random copolymers by RAFT polymerization with variation of fluorinated (F) groups; Bottom: lithographic patterning of “fluorozwitterist” on graphene through a chrome-coated photomask for spatial electronic tuning; Far-right: photograph of actual 1000-μm wide patterned stripes (blue) on graphene/SiO2/Si substrate (purple).

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    Figure 2

    Figure 2. (a) Chemical structures of homopolymers of 1,1,1,3,3,3-hexafluoro-2-propanol-substituted choline phosphate (HFIP-CP) and 1H,1H,2H,2H-perfluoro-1-octanol-substituted choline phosphate (PFO-CP), poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC), and poly(tridecafluoro-n-octyl methacrylate) (PTDFOMA); (b) Work function distributions derived from KPFM of homopolymers on graphene/SiO2/Si deposited from 1 mg/mL solutions; and (c) Illustration of hypothesized orientation of pendent FCP group with respect to graphene.

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    Figure 3

    Figure 3. Comparison of WF values of polymer-coated graphene measured by UPS (mean ± SD of n = 4 to 10 replicates).

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    Figure 4

    Figure 4. SFG spectra obtained from PMPC, HFIP-CP, PFO-CP, and PTDFOMA polymer thin films deposited on graphene/SiO2 (quartz) at (a) SSP and (b) PPP polarization combinations. The spectra are offset for clarity.

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    Figure 5

    Figure 5. Schematic diagram of the hypothesized orientations of (a) PMPC and (b) FCP polymer chains adsorbed on graphene based on SFG findings. For clarity, methylene groups in the pendant arm are drawn in brown. Pendant arm orientations with respect to graphene corroborate the measured work function shifts.

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    Figure 6

    Figure 6. (a,e) Optical microscopy of HFIP-CP and PFO-CP copolymers with 50 μm patterned stripes on graphene/SiO2/Si substrate. Kelvin probe force microscopy of the interface between patterned stripes and bare graphene showing the (b,f) height and (c,g) work function images with corresponding (d,h) line profiles. Both polymers induced p-doping relative to bare graphene surface.

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    Figure 7

    Figure 7. Top: Schematic diagram for copatterning polymeric fluorozwitterist (FCP) and zwitterist (PSBMA) on the same graphene substrate via sequential UV lithography; Bottom: XPS survey spectra of individual patterned polymer stripes (blue) on graphene (purple) indicated on the photograph with relevant peaks corresponding to the dipole-doping moieties.

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    Figure 8

    Figure 8. Co-patterned HFIP-CP (fluorozwitterist) and PSBMA (zwitterist) copolymers: KPFM of the interface between patterned stripes and bare graphene showing the (a,d) height and (b,e) work function images with corresponding (c,f) line profiles. The patterned fluorozwitterist and zwitterist induced p- and n-doping, respectively, relative to neighboring bare graphene surface.

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    7. 7
      Zhang, Z.; Huang, H.; Yang, X.; Zang, L. Tailoring Electronic Properties of Graphene by π-π Stacking with Aromatic Molecules. J. Phys. Chem. Lett. 2011, 2 (22), 2897– 2905,  DOI: 10.1021/jz201273r
      7
      Tailoring Electronic Properties of Graphene by π-π Stacking with Aromatic Molecules
      Zhang, Zengxing; Huang, Helin; Yang, Xiaomei; Zang, Ling
      Journal of Physical Chemistry Letters (2011), 2 (22), 2897-2905CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Intrinsic graphene is a semimetal or zero bandgap semiconductor, which hinders its applications for nanoelectronics. To develop high-performance nanodevices with graphene, it is necessary to open the bandgap and precisely control the charge carrier type and d. In this perspective, the authors focus on tailoring the electronic properties of graphene by noncovalent stacking with arom. mols. through π-π interaction. Different types of mols. (functioning as either an electron donor or acceptor when stacked with graphene) as reported in recent literature are presented regarding surface patterning, bandgap engineering, surface doping, as well as applications in nanodevices, particularly the field-effect transistors (FETs). From the current progress along this research line, future issues and challenges are also briefly discussed.
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    8. 8
      Gui, G.; Li, J.; Zhong, J. Band Structure Engineering of Graphene by Strain: First-Principles Calculations. Phys. Rev. B 2008, 78 (7), 075435,  DOI: 10.1103/PhysRevB.78.075435
      8
      Band structure engineering of graphene by strain: First-principles calculations
      Gui, Gui; Li, Jin; Zhong, Jianxin
      Physical Review B: Condensed Matter and Materials Physics (2008), 78 (7), 075435/1-075435/6CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      We have investigated the electronic structure of graphene under different planar strain distributions using the first-principles pseudopotential plane-wave method and the tight-binding approach. We found that graphene with a sym. strain distribution is always a zero band-gap semiconductor and its pseudogap decreases linearly with the strain strength in the elastic regime. However, asym. strain distributions in graphene result in opening of band gaps at the Fermi level. For the graphene with a strain distribution parallel to C-C bonds, its band gap continuously increases to its max. width of 0.486 eV as the strain increases up to 12.2%. For the graphene with a strain distribution perpendicular to C-C bonds, its band gap continuously increases only to its max. width of 0.170 eV as the strain increases up to 7.3%. The anisotropic nature of graphene is also reflected by different Poisson ratios under large strains in different directions. We found that the Poisson ratio approaches to a const. of 0.1732 under small strains but decreases differently under large strains along different directions.
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    9. 9
      Yu, Y.-J.; Zhao, Y.; Ryu, S.; Brus, L. E.; Kim, K. S.; Kim, P. Tuning the Graphene Work Function by Electric Field Effect. Nano Lett. 2009, 9 (10), 3430– 3434,  DOI: 10.1021/nl901572a
      9
      Tuning the graphene work function by electric field effect
      Yu, Young-Jun; Zhao, Yue; Ryu, Sunmin; Brus, Louis E.; Kim, Kwang S.; Kim, Philip
      Nano Letters (2009), 9 (10), 3430-3434CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      The authors report variation of the work function for single and bilayer graphene devices measured by scanning Kelvin probe microscopy (SKPM). By use of the elec. field effect, the work function of graphene can be adjusted as the gate voltage tunes the Fermi level across the charge neutrality point. Upon biasing the device, the surface potential map obtained by SKPM provides a reliable way to measure the contact resistance of individual electrodes contacting graphene.
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    10. 10
      Selhorst, R. C.; Puodziukynaite, E.; Dewey, J. A.; Wang, P.; Barnes, M. D.; Ramasubramaniam, A.; Emrick, T. Tetrathiafulvalene-Containing Polymers for Simultaneous Non-Covalent Modification and Electronic Modulation of MoS2 Nanomaterials. Chem. Sci. 2016, 7 (7), 4698– 4705,  DOI: 10.1039/C6SC00305B
      10
      Tetrathiafulvalene-containing polymers for simultaneous non-covalent modification and electronic modulation of MoS2 nanomaterials
      Selhorst, Ryan C.; Puodziukynaite, Egle; Dewey, Jeffrey A.; Wang, Peijian; Barnes, Michael D.; Ramasubramaniam, Ashwin; Emrick, Todd
      Chemical Science (2016), 7 (7), 4698-4705CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      Transition metal dichalcogenides (TMDCs) such as MoS2 comprise an important class of 2D semiconductors with numerous interesting electronic and mech. features. Full utilization of TMDCs in materials and devices, however, necessitates robust functionalization methods. We report well-defined tetrathiafulvalene (TTF)-based polymers, exploiting synthetic routes that overcome challenges previously assocd. with these systems. These platforms enable basal plane coordinative interactions with MoS2, conceptually in parallel with pyrene-contg. platforms for graphene and carbon nanotube modification. Not yet reported for TMDCs, these non-covalent interactions are universal and effective for MoS2 irresp. of the lattice structure, affording significantly enhanced soln. stabilization of the nanosheets. Addnl., the TTF-functionalized polymers offer electronic structure modulation of MoS2 by ground state charge transfer and work function redn., demonstrated using Kelvin probe force microscopy (KPFM). Notably, coordination and electronic effects are amplified for the TTF-polymers over TTF itself. Expts. are supported by first-principles d. functional theory (DFT) calcns. that probe polymer-TTF surface interactions with MoS2 and the resultant impact on electronic properties.
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    11. 11
      Selhorst, R.; Wang, P.; Barnes, M.; Emrick, T. Bithiazolidinylidene Polymers: Synthesis and Electronic Interactions with Transition Metal Dichalcogenides. Chem. Sci. 2018, 9 (22), 5047– 5051,  DOI: 10.1039/C8SC01416G
      11
      Bithiazolidinylidene polymers: synthesis and electronic interactions with transition metal dichalcogenides
      Selhorst, Ryan; Wang, Peijian; Barnes, Michael; Emrick, Todd
      Chemical Science (2018), 9 (22), 5047-5051CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      We describe the synthesis of electron acceptors consisting of bithiazolidinylidene (BT) derivs. incorporated into soln. processible polymers. Novel BT-contg. polymers displayed p-doping behavior when in contact with the n-type transition metal dichalcogenide (TMDC) MoS2. A work function (WF) increase of MoS2, resulting from contact with BT polymers, was measured by Kelvin probe force microscopy (KPFM), representing the first example of polymer p-doping of MoS2, which is beneficial for advancing the design of electronically tailored TMDCs.
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    12. 12
      Domercq, B.; Hreha, R. D.; Zhang, Y.-D.; Larribeau, N.; Haddock, J. N.; Schultz, C.; Marder, S. R.; Kippelen, B. Photo-Patternable Hole-Transport Polymers for Organic Light-Emitting Diodes. Chem. Mater. 2003, 15 (7), 1491– 1496,  DOI: 10.1021/cm020862u
      There is no corresponding record for this reference.
    13. 13
      Zakhidov, A. A.; Lee, J.-K.; DeFranco, J. A.; Fong, H. H.; Taylor, P. G.; Chatzichristidi, M.; Ober, C. K.; Malliaras, G. G. Orthogonal Processing: A New Strategy for Organic Electronics. Chem. Sci. 2011, 2 (6), 1178– 1182,  DOI: 10.1039/c0sc00612b
      13
      Orthogonal processing: A new strategy for organic electronics
      Zakhidov, Alexander A.; Lee, Jin-Kyun; DeFranco, John A.; Fong, Hon Hang; Taylor, Priscilla G.; Chatzichristidi, Margarita; Ober, Christopher K.; Malliaras, George G.
      Chemical Science (2011), 2 (6), 1178-1182CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      The concept of chem. orthogonality has long been practiced in the field of inorg. semiconductor fabrication, where it is necessary to deposit and remove a layer of photoresist without damaging the underlying layers. However, these processes involving light sensitive polymers often damage org. materials, preventing the use of photolithog. to pattern org. electronic devices. In this article we show that new photoresist materials that are orthogonal to orgs. allow the fabrication of complex devices, such as hybrid org./inorg. circuitry and full-color org. displays. The examples demonstrate that properly designed photoresists enable the fabrication of org. electronic devices using existing infrastructure.
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    14. 14
      Yang, R.; Wu, H.; Cao, Y.; Bazan, G. C. Control of Cationic Conjugated Polymer Performance in Light Emitting Diodes by Choice of Counterion. J. Am. Chem. Soc. 2006, 128 (45), 14422– 14423,  DOI: 10.1021/ja063723c
      14
      Control of Cationic Conjugated Polymer Performance in Light Emitting Diodes by Choice of Counterion
      Yang, Renqiang; Wu, Hongbin; Cao, Yong; Bazan, Guillermo C.
      Journal of the American Chemical Society (2006), 128 (45), 14422-14423CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Counterion exchange in cationic conjugated polyelectrolytes provides for a straightforward method to modulate the performance of these materials as the electron transport (injection) layer (ETL) in org. light emitting diodes. The bromide counterions of [(9,9-bis(6'-N,N,N-trimethylammonium)-hexyl)fluorene]bromide (PF-Br) can be easily replaced with trifluoromethylsulfonate (PF-CF3SO3), tetrakis(imidazolyl)borate (PF-BIm4) or tetrakis(3,5-trifluoromethylphenyl)borate (PF-BArF4) by a procedure that involves pptn. and washing. The performance of LEDs using MEH-PPV as the emissive layer, Al as the cathode, and the conjugated polyelectrolytes as the ETL varies in the order: PF-BIm4 > PF-CF3SO3 > PF-Br > PF-BArF4. In the case of PF-BIm4, the luminous efficiencies of the devices are similar to those of devices using Ba as the cathode. Thus, by properly choosing the counterion one can use higher work function metals that are more stable than lower work function metals without a substantial barrier to electron injection.
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    15. 15
      Zhang, Y.-Q.; Lin, H.-A.; Pan, Q.-C.; Qian, S.-H.; Zhang, S.-H.; Qiu, G.; Luo, S.-C.; Yu, H.; Zhu, B. Tunable Protein/Cell Binding and Interaction with Neurite Outgrowth of Low-Impedance Zwitterionic PEDOTs. ACS Appl. Mater. Interfaces 2020, 12 (10), 12362– 12372,  DOI: 10.1021/acsami.9b23025
      15
      Tunable Protein/Cell Binding and Interaction with Neurite Outgrowth of Low-Impedance Zwitterionic PEDOTs
      Zhang, Ya-Qiong; Lin, Hsing-An; Pan, Qi-Chao; Qian, Si-Hao; Zhang, Shu-Hua; Qiu, Gao; Luo, Shyh-Chyang; Yu, Hsiao-hua; Zhu, Bo
      ACS Applied Materials & Interfaces (2020), 12 (10), 12362-12372CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Zwitterionic poly(3,4-ethylenedioxythiophene) (PEDOT) is an effective electronic material for bioelectronics because it exhibits efficient elec. trade-off and diminishes immune response. To promote the use of zwitterionic PEDOTs in bioelectronic devices, esp. for cell alignment control and close electrocoupling, features such as tunable interaction of PEDOTs with proteins/cells and spatially modulating cell behavior are required. However, there is a lack of reliable methods to assemble zwitterionic EDOTs with other functionalized EDOT materials, having different polarities and oxidn. potentials, to prep. PEDOTs with the aforementioned surface properties. In this study, the authors have developed a surfactant-assisted electropolymn. to assemble phosphorylcholine (PC)-functionalized EDOT with other functionalized EDOTs. By adjusting compns., the interaction of PEDOT copolymers with proteins/cells can be finely tuned; the compn. adjustment has an ignorable influence on the impedance of the copolymers. The authors also demonstrate that the cell-repulsive force generated from PC can spatially guide the neurite outgrowth to form a neuron network at single-cell resoln. and greatly enhance the neurite outgrowth by 179%, which is significantly more distinctive than the reported topog. effect. The authors expect that the derived tunable protein/cell interaction and the PC-induced repulsive guidance for the neurite outgrowth can make low-impedance zwitterionic PEDOTs more useful in bioelectronics.
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    16. 16
      Liu, Y.; Duzhko, V. V.; Page, Z. A.; Emrick, T.; Russell, T. P. Conjugated Polymer Zwitterions: Efficient Interlayer Materials in Organic Electronics. Acc. Chem. Res. 2016, 49 (11), 2478– 2488,  DOI: 10.1021/acs.accounts.6b00402
      16
      Conjugated Polymer Zwitterions: Efficient Interlayer Materials in Organic Electronics
      Liu, Yao; Duzhko, Volodimyr V.; Page, Zachariah A.; Emrick, Todd; Russell, Thomas P.
      Accounts of Chemical Research (2016), 49 (11), 2478-2488CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      A review. Conjugated polymer zwitterions (CPZs) are neutral, hydrophilic, polymer semiconductors. The pendent zwitterions, viewed as side chain dipoles, impart soly. in polar solvents for soln. processing, and open opportunities as interfacial components of optoelectronic devices, for example, between metal electrodes and org. semiconductor active layers. Such interlayers are crucial for defining the performance of org. electronic devices, e.g., field-effect transistors (OFETs), light-emitting diodes (OLEDs), and photovoltaics (OPVs), all of which consist of multilayer structures. The interlayers reduce the Schottky barrier height and thus improve charge injection in OFETs and OLEDs. In OPVs, the interlayers serve to increase the built-in elec. p.d. (Vbi) across the active layer, ensuring efficient extn. of photogenerated charge carriers. In general, polar and even charged electronically active polymers have gained recognition for their ability to modify metal/semiconductor interfaces to the benefit of org. electronics. While conjugated polyelectrolytes (CPEs) as interlayer materials are well-documented, open questions remain about the role of mobile counterions in CPE-contg. devices. CPZs possess the processing advantages of CPEs, but as neutral mols. lack any potential complications assocd. with counterions. The electronic implications of CPZs on metal electrodes stem from the orientation of the zwitterion dipole moment in close proximity to the metal surface, and the resultant surface-induced polarization. This generates an interfacial dipole (Δ) at the CPZ/metal interface, altering the work function of the electrode, as confirmed by UPS, and improving device performance. An ideal cathode interlayer would reduce electrode work function, have orthogonal processability to the active layer, exhibit good film forming properties (i.e., wettability/uniformity), prevent exciton quenching, possess optimal electron affinity that neither limits the work function redn. nor impedes the charge extn., transport electrons selectively, and exhibit long-term stability. The authors' recent discoveries show that CPZs achieve many of these attributes, and are poised for further expansion and development in the interfacial science of org. electronics. This Account reviews a recent collaboration that began with the synthesis of CPZs and a study of their structural and electronic properties on metals, then extended to their application as interlayer materials for OPVs. CPZ structure-property relations based on several material platforms, ranging from homopolymers to copolymers, and from materials with intrinsic p-type conjugated backbones to those with intrinsic n-type conjugated backbones are discussed. Key components of such interlayers, including (i) the origin of work function redn. of CPZ interlayers on metals; (ii) the role of the frontier MO energy levels and their trade-offs in optimizing electronic and device properties; and (iii) the role of polymer cond. type and the magnitude of charge carrier mobility are discussed. The authors' motivation is to present the authors' prior use and current understanding of CPZs as interlayer materials in org. electronics, and describe outstanding issues and future potential directions.
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    17. 17
      Liu, Y.; Sheri, M.; Cole, M. D.; Emrick, T.; Russell, T. P. Combining Fullerenes and Zwitterions in Non-Conjugated Polymer Interlayers to Raise Solar Cell Efficiency. Angew. Chem., Int. Ed. 2018, 57 (31), 9675– 9678,  DOI: 10.1002/anie.201803748
      17
      Combining Fullerenes and Zwitterions in Non-Conjugated Polymer Interlayers to Raise Solar Cell Efficiency
      Liu, Yao; Sheri, Madhu; Cole, Marcus D.; Emrick, Todd; Russell, Thomas P.
      Angewandte Chemie, International Edition (2018), 57 (31), 9675-9678CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Polymer zwitterions were synthesized by nucleophilic ring-opening of 3,3'-(but-2-ene-1,4-diyl)bis(1,2-oxathiolane 2,2-dioxide) (a bis-sultone) with functional perylene diimide (PDI) or fullerene monomers. Integration of these polymers into solar cell devices as cathode interlayers boosted efficiencies of fullerene-based org. photovoltaics (OPVs) from 2.75 % to 10.74 %, and of non-fullerene-based OPVs from 4.25 % to 10.10 %, demonstrating the versatility of these interlayer materials in OPVs. The fullerene-contg. polymer zwitterion (C60-PZ) showed a higher interfacial dipole (Δ) value and electron mobility than its PDI counterpart (PDI-PZ), affording solar cells with high efficiency. The power of PDI-PZ and C60-PZ to improve electron injection and extn. processes when positioned between metal electrodes and org. semiconductors highlights their promise to overcome energy barriers at the hard-soft materials interface of org. electronics.
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    18. 18
      Alon, H.; Stern, C.; Kirshner, M.; Sinai, O.; Wasserman, M.; Selhorst, R.; Gasper, R.; Ramasubramaniam, A.; Emrick, T.; Naveh, D. Lithographically Patterned Functional Polymer-Graphene Hybrids for Nanoscale Electronics. ACS Nano 2018, 12 (2), 1928– 1933,  DOI: 10.1021/acsnano.7b08844
      18
      Lithographically patterned functional polymer-graphene hybrids for nanoscale electronics
      Alon, Hadas; Stern, Chen; Kirshner, Moshe; Sinai, Ofer; Wasserman, Michal; Selhorst, Ryan; Gasper, Raymond; Ramasubramaniam, Ashwin; Emrick, Todd; Naveh, Doron
      ACS Nano (2018), 12 (2), 1928-1933CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Two-dimensional (2D) materials are believed to hold significant promise in nanoscale optoelectronics. While significant progress has been made in this field over the past decade, the ability to control charge carrier d. with high spatial precision remains an outstanding challenge in 2D devices. The authors present an approach that simultaneously addresses the dual issues of charge-carrier doping and spatial precision based on a functional lithog. resist that employs methacrylate polymers contg. zwitterionic sulfobetaine pendent groups for noncovalent surface doping of 2D materials. They demonstrate scalable approaches for patterning these polymer films via electron-beam lithog., achieving precise spatial control over carrier doping for fabrication of high-quality, all-2D, lateral p-n junctions in graphene. Their approach preserves all of the desirable structural and electronic properties of graphene while exclusively modifying its surface potential. The functional polymer resist platform and concept offers a facile route toward lithog. doping of graphene- and other 2D material-based optoelectronic devices.
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    19. 19
      Lee, H.; Puodziukynaite, E.; Zhang, Y.; Stephenson, J. C.; Richter, L. J.; Fischer, D. A.; DeLongchamp, D. M.; Emrick, T.; Briseno, A. L. Poly(Sulfobetaine Methacrylate)s as Electrode Modifiers for Inverted Organic Electronics. J. Am. Chem. Soc. 2015, 137 (1), 540– 549,  DOI: 10.1021/ja512148d
      19
      Poly(sulfobetaine methacrylate)s as Electrode Modifiers for Inverted Organic Electronics
      Lee, Hyunbok; Puodziukynaite, Egle; Zhang, Yue; Stephenson, John C.; Richter, Lee J.; Fischer, Daniel A.; DeLongchamp, Dean M.; Emrick, Todd; Briseno, Alejandro L.
      Journal of the American Chemical Society (2015), 137 (1), 540-549CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The authors demonstrate the use of poly(sulfobetaine methacrylate) (PSBMA), and its pyrene-contg. copolymer, as soln.-processable work function reducers for inverted org. electronic devices. A notable feature of PSBMA is its orthogonal soly. relative to solvents typically employed in the processing of org. semiconductors. A strong permanent dipole moment on the sulfobetaine moiety was calcd. by d. functional theory. PSBMA interlayers reduced the work function of metals, graphene, and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) by over 1 eV, and an ultrathin interlayer of PSBMA reduced the electron injection barrier between indium tin oxide (ITO) and C70 by 0.67 eV. As a result, the performance of org. photovoltaic devices with PSBMA interlayers is significantly improved, and enhanced electron injection is demonstrated in electron-only devices with ITO, PEDOT:PSS, and graphene electrodes. This work makes available a new class of dipole-rich, counterion-free, pH insensitive polymer interlayers with demonstrated effectiveness in inverted devices.
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    20. 20
      Pagaduan, J. N.; Hight-Huf, N.; Datar, A.; Nagar, Y.; Barnes, M.; Naveh, D.; Ramasubramaniam, A.; Katsumata, R.; Emrick, T. Electronic Tuning of Monolayer Graphene with Polymeric “Zwitterists.. ACS Nano 2021, 15 (2), 2762– 2770,  DOI: 10.1021/acsnano.0c08624
      20
      Electronic tuning of monolayer graphene with polymeric zwitterists
      Pagaduan, James Nicolas; Hight-Huf, Nicholas; Datar, Avdhoot; Nagar, Yehiel; Barnes, Michael; Naveh, Doron; Ramasubramaniam, Ashwin; Katsumata, Reika; Emrick, Todd
      ACS Nano (2021), 15 (2), 2762-2770CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Work function engineering of two-dimensional (2D) materials by application of polymer coatings represents a research thrust that promises to enhance the performance of electronic devices. While polymer zwitterions have been demonstrated to significantly modify the work function of both metal electrodes and 2D materials due to their dipole-rich structure, the impact of zwitterion chem. structure on work function modulation is not well understood. To address this knowledge gap, we synthesized a series of sulfobetaine-based zwitterionic random copolymers with variable substituents and used them in lithog. patterning for the prepn. of neg.-tone resists (i.e., zwitterists) on monolayer graphene. UPS indicated a significant work function redn., as high as 1.5 eV, induced by all polymer zwitterions when applied as ultrathin films (<10 nm) on monolayer graphene. Of the polymers studied, the piperidinyl-substituted version, produced the largest dipole normal to the graphene sheet, thereby inducing the max. work function redn. D. functional theory calcns. probed the influence of zwitterion compn. on dipole orientation, while lithog. patterning allowed for evaluation of surface potential contrast via Kelvin probe force microscopy. Overall, this polymer zwitterist design holds promise for fine-tuning 2D materials electronics with spatial control based on the chem. of the polymer coating and the dimensions of the lithog. patterning.
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    21. 21
      Hight-Huf, N.; Nagar, Y.; Levi, A.; Pagaduan, J. N.; Datar, A.; Katsumata, R.; Emrick, T.; Ramasubramaniam, A.; Naveh, D.; Barnes, M. D. Polarization-Driven Asymmetric Electronic Response of Monolayer Graphene to Polymer Zwitterions Probed from Both Sides. ACS Appl. Mater. Interfaces 2021, 13, 47945,  DOI: 10.1021/acsami.1c13505
      21
      Polarization-Driven Asymmetric Electronic Response of Monolayer Graphene to Polymer Zwitterions Probed from Both Sides
      Hight-Huf, Nicholas; Nagar, Yehiel; Levi, Adi; Pagaduan, James Nicolas; Datar, Avdhoot; Katsumata, Reika; Emrick, Todd; Ramasubramaniam, Ashwin; Naveh, Doron; Barnes, Michael D.
      ACS Applied Materials & Interfaces (2021), 13 (40), 47945-47953CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      We investigated the nature of graphene surface doping by zwitterionic polymers and the implications of weak in-plane and strong through-plane screening using a novel sample geometry that allows direct access to either the graphene or the polymer side of a graphene/polymer interface. Using both Kelvin probe and electrostatic force microscopies, we obsd. a significant upshift in the Fermi level in graphene of ~ 260 meV that was dominated by a change in polarizability rather than pure charge transfer with the org. overlayer. This phys. picture is supported by d. functional theory (DFT) calcns., which describe a redistribution of charge in graphene in response to the dipoles of the adsorbed zwitterionic moieties, analogous to a local DC Stark effect. Strong metallic-like screening of the adsorbed dipoles was obsd. by employing an inverted geometry, an effect identified by DFT to arise from a strongly asym. redistribution of charge confined to the side of graphene proximal to the zwitterion dipoles. Transport measurements confirm n-type doping with no significant impact on carrier mobility, thus demonstrating a route to desirable electronic properties in devices that combine graphene with lithog. patterned polymers.
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    22. 22
      Zhou, L.; Triozzi, A.; Figueiredo, M.; Emrick, T. Fluorinated Polymer Zwitterions: Choline Phosphates and Phosphorylcholines. ACS Macro Lett. 2021, 10 (10), 1204– 1209,  DOI: 10.1021/acsmacrolett.1c00451
      22
      Fluorinated Polymer Zwitterions: Choline Phosphates and Phosphorylcholines
      Zhou, Le; Triozzi, Alexandria; Figueiredo, Marxa; Emrick, Todd
      ACS Macro Letters (2021), 10 (10), 1204-1209CODEN: AMLCCD; ISSN:2161-1653. (American Chemical Society)
      Among zwitterionic structures, the choline phosphate (CP) group is uniquely attractive for its ability to access novel chem. compns. that embed functional groups directly into the zwitterionic moiety. This paper describes the attachment of fluorinated alkyl groups to CP moieties, yielding zwitterionic monomers 1 and 2 that proved amenable to controlled free radical polymn. and the prodn. of a new set of CP-contg. fluorinated polymers and copolymers with phosphorylcholine (PC) zwitterions. This combination of fluorinated hydrocarbons and zwitterions affords novel, water-sol. polymeric amphiphiles that we have examd. at fluid interfaces, as coatings, in cell culture, and in magnetic resonance imaging.
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    23. 23
      Zhou, L.; Yang, Z.; Pagaduan, J. N.; Emrick, T. Fluorinated Zwitterionic Polymers as Dynamic Surface Coatings. Polym. Chem. 2022, 14 (1), 32– 36,  DOI: 10.1039/D2PY01197B
      There is no corresponding record for this reference.
    24. 24
      Borkar, S.; Jankova, K.; Siesler, H. W.; Hvilsted, S. New Highly Fluorinated Styrene-Based Materials with Low Surface Energy Prepared by ATRP. Macromolecules 2004, 37 (3), 788– 794,  DOI: 10.1021/ma034952b
      24
      New highly fluorinated styrene-based materials with low surface energy prepared by ATRP
      Borkar, Sachin; Jankova, Katja; Siesler, Heinz W.; Hvilsted, Soren
      Macromolecules (2004), 37 (3), 788-794CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
      2,3,5,6-Tetrafluoro-4-(2,2,3,3,3-pentafluoropropoxy)styrene [TF(F5)S] and 2,3,5,6-tetrafluoro-4-(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyloxy)styrene [TF(F15)S] are prepd. by nucleophilic substitution of 2,3,4,5,6-pentafluorostyrene. The neat monomers are subjected to atom transfer radical polymn. (ATRP) at 110°C to high conversions in relatively short times (10-120 min); TF(F5)S is addnl. polymd. at 70 and 90°C. Block copolymers with styrene are prepd. by the macroinitiator approach. All polymers, in the no.-av. mol. wt. range from 6000 to 35,000, have polydispersity indexes between 1.08 and 1.37. The homopolymers show glass transitions from 16 to 62°C depending on mol. wt., whereas the block copolymers exhibit phase sepn. mirrored in two glass temps., which could be obsd. when the smallest block constitutes more than 10 mol%. The fluorinated side chains of P(TF(F5)S) and P(TF(F15)S) enrich the surface of thin films, which results in an advancing water contact angle of 117° and 122°, resp. Both XPS analyses and contact angle measurements strongly imply that the fluorinated parts of the block copolymers migrate to the surface and create low surface energy films.
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    25. 25
      Ming, W.; Tian, M.; van de Grampel, R. D.; Melis, F.; Jia, X.; Loos, J.; van der Linde, R. Low Surface Energy Polymeric Films from Solventless Liquid Oligoesters and Partially Fluorinated Isocyanates. Macromolecules 2002, 35 (18), 6920– 6929,  DOI: 10.1021/ma020650i
      25
      Low Surface Energy Polymeric Films from Solventless Liquid Oligoesters and Partially Fluorinated Isocyanates
      Ming, W.; Tian, M.; van de Grampel, R. D.; Melis, F.; Jia, X.; Loos, J.; van der Linde, R.
      Macromolecules (2002), 35 (18), 6920-6929CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
      Partially fluorinated isocyanates were synthesized from hexamethylene diisocyanate (HDI) and an HDI trimer (Desmodur N 3300). The perfluoroalkyl group (Rf) was C6F13 or C8F17, and the ratio between Rf and the isocyanate group (NCO) was 1/99 in the case of HDI or 1/49 in the case of N 3300. Polymeric films with surface energies as low as 10 mN/m were obtained from mixts. of these partially fluorinated isocyanates and a previously reported hydroxyl-end-capped solventless liq. oligoester. Contact angles of water and hexadecane reached 120° and 80°, resp., when less than 1 wt.% of fluorine was present in the films. The surface enrichment of fluorine-contg. species was confirmed by XPS investigations. At a fluorine concn. of 0.5-1.0 wt.%, the surface F/C at. ratio (at a 15° takeoff angle) was greater than 1; the surface enrichment factor of fluorine was up to 600. The topol. structures of the polymeric films were recorded by an at. force microscope under tapping mode. While the height images indicated that the surface was smooth at the nanometer scale in a 1 μm × 1 μm area, the phase images revealed that fluorine-enriched domains were present at the surface. As the fluorine concn. increased, the fluorine-enriched domains grew from tiny spots (2-3 nm) to larger round domains (15-25 nm in diam.). The low surface energies of the films could be ascribed to the strong surface segregation of fluorinated species.
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    26. 26
      Hight-Huf, N.; Pagaduan, J. N.; Katsumata, R.; Emrick, T.; Barnes, M. D. Stabilization of Three-Particle Excitations in Monolayer MoS2 by Fluorinated Methacrylate Polymers. J. Phys. Chem. Lett. 2022, 13 (21), 4794– 4799,  DOI: 10.1021/acs.jpclett.2c01150
      There is no corresponding record for this reference.
    27. 27
      Saha, S.; Samanta, P.; Murmu, N. C.; Banerjee, A.; Ganesh, R. S.; Inokawa, H.; Kuila, T. Modified Electrochemical Charge Storage Properties of H-BN/rGO Superlattice through the Transition from n to p Type Semiconductor by Fluorine Doping. Chemical Engineering Journal 2018, 339, 334– 345,  DOI: 10.1016/j.cej.2018.01.141
      There is no corresponding record for this reference.
    28. 28
      Melitz, W.; Shen, J.; Kummel, A. C.; Lee, S. Kelvin Probe Force Microscopy and Its Application. Surf. Sci. Rep. 2011, 66 (1), 1– 27,  DOI: 10.1016/j.surfrep.2010.10.001
      There is no corresponding record for this reference.
    29. 29
      Wang, P.; Selhorst, R.; Emrick, T.; Ramasubramaniam, A.; Barnes, M. D. Bidirectional Electronic Tuning of Single-Layer MoS2 with Conjugated Organochalcogens. J. Phys. Chem. C 2019, 123 (2), 1506– 1511,  DOI: 10.1021/acs.jpcc.8b10826
      29
      Bidirectional Electronic Tuning of Single-Layer MoS2 with Conjugated Organochalcogens
      Wang, Peijian; Selhorst, Ryan; Emrick, Todd; Ramasubramaniam, Ashwin; Barnes, Michael D.
      Journal of Physical Chemistry C (2019), 123 (2), 1506-1511CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      We report a bidirectional tuning of the electronic properties of single-layer molybdenum disulfide (MoS2) by n-doping with the electron donating tetrathiafulvalene (TTF) and p-doping with the electron accepting bithiazolidinylidene (BT). Using Kelvin probe force microscopy (KPFM), we spatially monitored changes in the work function of monolayer MoS2 on silicon oxide (SiO2) and sapphire (Al2O3). KPFM, in conjunction with spectroscopic characterization, showed MoS2 work function shifts as significant as 1.24 eV for TTF doping, and 0.43 eV for BT doping, when Al2O3 was employed as the underlying substrate. Less dramatic changes were obsd. for MoS2 on SiO2/Si, revealing a significant impact of substrate selection on the electronic properties of this 2D material. High-level computations helped guide expts. on chem. modulation of the electronic properties of this transition-metal dichalcogenide.
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    30. 30
      Wang, X.; Xu, J.-B.; Xie, W.; Du, J. Quantitative Analysis of Graphene Doping by Organic Molecular Charge Transfer. J. Phys. Chem. C 2011, 115 (15), 7596– 7602,  DOI: 10.1021/jp200386z
      30
      Quantitative Analysis of Graphene Doping by Organic Molecular Charge Transfer
      Wang, Xiaomu; Xu, Jian-Bin; Xie, Weiguang; Du, Jun
      Journal of Physical Chemistry C (2011), 115 (15), 7596-7602CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Both n-type and p-type doped exfoliated graphene sheets are presented by virtue of adsorbing org. mols. Flat org. layers are uniformly grown on graphene sheets by the technique. Meanwhile, the high-mobility attribute of graphene is largely preserved, as adsorption on the graphene surface does not significantly modify the π-bonding networks of graphene. By employing Kelvin probe force microscopy, the 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) mols. obtain electrons from graphene whereas vanadyl-phthalocyanine (VOPc) mols. donate electrons to it. The amt. of charge transfer by F4-TCNQ and VOPc to bilayer graphene on silicon dioxide substrate is ∼0.4 and 0.1 electron/mol. by a tight-binding self-consistent model, resp. The consistent theor. and doping of graphene by org. mol. charge transfer has great implications for future large-scale applications of graphene-based nanoelectronics.
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    31. 31
      Zhou, X.; He, S.; Brown, K. A.; Mendez-Arroyo, J.; Boey, F.; Mirkin, C. A. Locally Altering the Electronic Properties of Graphene by Nanoscopically Doping It with Rhodamine 6G. Nano Lett. 2013, 13 (4), 1616– 1621,  DOI: 10.1021/nl400043q
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      Locally Altering the Electronic Properties of Graphene by Nanoscopically Doping It with Rhodamine 6G
      Zhou, Xiaozhu; He, Shu; Brown, Keith A.; Mendez-Arroyo, Jose; Boey, Freddy; Mirkin, Chad A.
      Nano Letters (2013), 13 (4), 1616-1621CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Rhodamine 6G (R6G), patterned by dip-pen nanolithog. on graphene, can be used to locally n-dope it in a controlled fashion. The authors study the transport and assembly properties of R6G on graphene and show that in general the π-π stacking between the arom. components of R6G and the underlying graphene drives the assembly of these mols. onto the underlying substrate. However, two distinct transport and assembly behaviors, dependent upon the presence or absence of R6G dimers, were identified. In particular, at high concns. of R6G on the tip, dimers are transferred to the substrate and form contiguous and stable lines, while at low concns., the R6G is transferred as monomers and forms patchy, unstable, and relatively ill-defined features. Finally, Kelvin probe force microscopy expts. show that the local electrostatic potential of the graphene changes as function of modification with R6G; this behavior is consistent with local mol. doping, highlighting a path for controlling the electronic properties of graphene with nanoscale resoln.
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    32. 32
      Leckey, R. Ultraviolet Photoelectron Spectroscopy of Solids. In Surface Analysis Methods in Materials Science; O’Connor, D. J., Sexton, B. A., Smart, R. St. C., Eds.; Springer Series in Surface Sciences; Springer: Berlin, Heidelberg, 2003; pp 337– 345.  DOI: 10.1007/978-3-662-05227-3_14 .
      There is no corresponding record for this reference.
    33. 33
      Lee, W. H.; Suk, J. W.; Lee, J.; Hao, Y.; Park, J.; Yang, J. W.; Ha, H.-W.; Murali, S.; Chou, H.; Akinwande, D.; Kim, K. S.; Ruoff, R. S. Simultaneous Transfer and Doping of CVD-Grown Graphene by Fluoropolymer for Transparent Conductive Films on Plastic. ACS Nano 2012, 6 (2), 1284– 1290,  DOI: 10.1021/nn203998j
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      Simultaneous Transfer and Doping of CVD-Grown Graphene by Fluoropolymer for Transparent Conductive Films on Plastic
      Lee, Wi Hyoung; Suk, Ji Won; Lee, Jongho; Hao, Yufeng; Park, Jaesung; Yang, Jae Won; Ha, Hyung-Wook; Murali, Shanthi; Chou, Harry; Akinwande, Deji; Kim, Kwang S.; Ruoff, Rodney S.
      ACS Nano (2012), 6 (2), 1284-1290CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Chem. doping can decrease sheet resistance of graphene while maintaining its high transparency. The authors report a new method to simultaneously transfer and dope CVD grown graphene onto a target substrate using a fluoropolymer as both the supporting and doping layer. Solvent was used to remove a significant fraction of the supporting fluoropolymer, but residual polymer remained that doped the graphene significantly. This contrasts with a more widely used supporting layer, polymethylmethacrylate, which does not induce significant doping during transfer. The fluoropolymer doping mechanism can be explained by the rearrangement of fluorine atoms on the graphene basal plane caused by either thermal annealing or soaking in solvent, which induces ordered dipole moments near the graphene surface. This simultaneous transfer and doping of the graphene with a fluoropolymer increases the carrier d. significantly, and the resulting monolayer graphene film exhibits a sheet resistance of ∼320 Ω/sq. Finally, the method presented here was used to fabricate flexible and a transparent graphene electrode on a plastic substrate.
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    34. 34
      Liu, F.; Page, Z. A.; Duzhko, V. V.; Russell, T. P.; Emrick, T. Conjugated Polymeric Zwitterions as Efficient Interlayers in Organic Solar Cells. Adv. Mater. 2013, 25 (47), 6868– 6873,  DOI: 10.1002/adma.201302477
      34
      Conjugated polymeric zwitterions as efficient interlayers in organic solar cells
      Liu, Feng; Page, Zachariah A.; Duzhko, Volodimyr V.; Russell, Thomas P.; Emrick, Todd
      Advanced Materials (Weinheim, Germany) (2013), 25 (47), 6868-6873CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Inserting conjugated polymeric zwitterions between the metal cathode and active PTB7/PC71BM layer resulted in a PCE increase of >500% (0.92% to 5.78% for Ag-cathode devices, and 68% (4.39% to 7.36%) for Al-contg. devices.
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    35. 35
      Crispin, X.; Geskin, V.; Crispin, A.; Cornil, J.; Lazzaroni, R.; Salaneck, W. R.; Brédas, J.-L. Characterization of the Interface Dipole at Organic/ Metal Interfaces. J. Am. Chem. Soc. 2002, 124 (27), 8131– 8141,  DOI: 10.1021/ja025673r
      35
      Characterization of the Interface Dipole at Organic/ Metal Interfaces
      Crispin, Xavier; Geskin, Victor; Crispin, Annica; Cornil, Jerome; Lazzaroni, Roberto; Salaneck, William R.; Bredas, Jean-Luc
      Journal of the American Chemical Society (2002), 124 (27), 8131-8141CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      In orgs.-based (opto)electronic devices, the interface dipoles formed at the org./metal interfaces play a key role in detg. the barrier for charge (hole or electron) injection between the metal electrodes and the active org. layers. The origin of this dipole is rationalized here from the results of a joint exptl. and theor. study based on the interaction between acrylonitrile, a π-conjugated mol., and transition metal surfaces (Cu, Ni, and Fe). The adsorption of acrylonitrile on these surfaces was studied exptl. by photoelectron spectroscopies, while quantum mech. methods based on d. functional theory were used to study the systems theor. Apparently the interface dipole formed at an org./metal interface can be divided into two contributions: (i) the 1st corresponds to the chem. dipole induced by a partial charge transfer between the org. layers and the metal upon chemisorption of the org. mols. on the metal surface, and (ii) the 2nd relates to the change in metal surface dipole because of the modification of the metal electron d. tail that is induced by the presence of the adsorbed org. mols. The authors' anal. shows that the charge injection barrier in devices can be tuned by modulating various parameters: the chem. potential of the bare metal (given by its work function), the metal surface dipole, and the ionization potential and electron affinity of the org. layer.
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    36. 36
      Terrones, H.; Lv, R.; Terrones, M.; Dresselhaus, M. S. The Role of Defects and Doping in 2D Graphene Sheets and 1D Nanoribbons. Rep. Prog. Phys. 2012, 75 (6), 062501,  DOI: 10.1088/0034-4885/75/6/062501
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      The role of defects and doping in 2D graphene sheets and 1D nanoribbons
      Terrones, Humberto; Lv, Ruitao; Terrones, Mauricio; Dresselhaus, Mildred S.
      Reports on Progress in Physics (2012), 75 (6), 062501/1-062501/30CODEN: RPPHAG; ISSN:0034-4885. (Institute of Physics Publishing)
      A review. Defects are usually seen as imperfections in materials that could significantly degrade their performance. However, at the nanoscale, defects could be extremely useful since they could be exploited to generate novel, innovative and useful materials and devices. Graphene and graphene nanoribbons are no exception. This review therefore tries to categorize defects, emphasize their importance, introduce the common routes to study and identify them and to propose new ways to construct novel devices based on 'defective' graphene-like materials. In particular, we will discuss defects in graphene-like systems including (a) structural (sp2-like) defects, (b) topol. (sp2-like) defects, (c) doping or functionalization (sp2- and sp3-like) defects and (d) vacancies/edge type defects (non-sp2-like). It will be demonstrated that defects play a key role in graphene physicochem. properties and could even be crit. to generate biocompatible materials. There are numerous challenges in this emerging field, and we intend to provide a stimulating account which could trigger new science and technol. developments based on defective graphene-like materials that could be introduced into other at. layered materials, such as BN, MoS2 and WS2, not discussed in this review.
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    37. 37
      McDonnell, S.; Addou, R.; Buie, C.; Wallace, R. M.; Hinkle, C. L. Defect-Dominated Doping and Contact Resistance in MoS2. ACS Nano 2014, 8 (3), 2880– 2888,  DOI: 10.1021/nn500044q
      37
      Defect-Dominated Doping and Contact Resistance in MoS2
      McDonnell, Stephen; Addou, Rafik; Buie, Creighton; Wallace, Robert M.; Hinkle, Christopher L.
      ACS Nano (2014), 8 (3), 2880-2888CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Achieving low resistance contacts is vital for the realization of nanoelectronic devices based on transition metal dichalcogenides. Intrinsic defects in MoS2 dominate the metal/MoS2 contact resistance and provide a low Schottky barrier independent of metal contact work function. Also, MoS2 can exhibit both n-type and p-type conduction at different points on a same sample. The authors identify these regions independently by complementary characterization techniques and show how the Fermi level can shift by 1 eV over tens of nanometers in spatial resoln. These variations in doping are defect-chem.-related and are independent of contact metal. This raises questions on previous reports of metal-induced doping of MoS2 since the same metal in contact with MoS2 can exhibit both n- and p-type behavior. These results may provide a potential route for achieving low electron and hole Schottky barrier contacts with a single metal deposition.
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    38. 38
      Chowdhury, A. U.; Chang, D.; Xu, Y.; Hong, K.; Sumpter, B. G.; Carrillo, J.-M. Y.; Doughty, B. Mapping the Interfacial Chemistry and Structure of Partially Fluorinated Bottlebrush Polymers and Their Linear Analogues. Langmuir 2021, 37 (1), 211– 218,  DOI: 10.1021/acs.langmuir.0c02786
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      Mapping the Interfacial Chemistry and Structure of Partially Fluorinated Bottlebrush Polymers and Their Linear Analogues
      Chowdhury, Azhad U.; Chang, Dongsook; Xu, Yuewen; Hong, Kunlun; Sumpter, Bobby G.; Carrillo, Jan-Michael Y.; Doughty, Benjamin
      Langmuir (2021), 37 (1), 211-218CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      Polymer interfaces are key to a range of applications including membranes for chem. sepns., hydrophobic coatings, and passivating layers for antifouling. While important, challenges remain in probing the interfacial monolayer where the mol. ordering and orientation can change depending on the chem. makeup or processing conditions. In this work, we leverage surface specific vibrational sum frequency generation (SFG) and the assocd. dependence on mol. symmetry to elucidate the ordering and orientations of key functional groups for poly(2,2,2-trifluoroethyl methacrylate) bottlebrush polymers and their linear polymer analogs. These measurements were framed by atomistic mol. dynamic simulations to provide a complementary phys. picture of the gas-polymer interface. Simulations and SFG measurements show that methacrylate backbones are buried beneath a layer of trifluoroethyl contg. side groups that result in structurally similar interfaces regardless of the polymer mol. wt. or architecture. The av. orientational angles of the trifluoroethyl contg. side groups differ depending on polymer linear and bottlebrush architectures, suggesting that the surface groups can reorient via available rotational degrees of freedom. Results show that the surfaces of the bottlebrush and linear polymer samples do not strongly depend on mol. wt. or architecture. As such, one cannot rely on increasing the mol. wt. or altering the architecture to tune surface properties. This insight into the polymer interfacial structure is expected to advance the design of new material interfaces with tailored chem./functional properties.
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    39. 39
      Tateishi, Y.; Kai, N.; Noguchi, H.; Uosaki, K.; Nagamura, T.; Tanaka, K. Local Conformation of Poly(Methyl Methacrylate) at Nitrogen and Water Interfaces. Polym. Chem. 2010, 1 (3), 303– 311,  DOI: 10.1039/B9PY00227H
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      Local conformation of poly(methyl methacrylate) at nitrogen and water interfaces
      Tateishi, Yohei; Kai, Naoki; Noguchi, Hidenori; Uosaki, Kohei; Nagamura, Toshihiko; Tanaka, Keiji
      Polymer Chemistry (2010), 1 (3), 303-311CODEN: PCOHC2; ISSN:1759-9962. (Royal Society of Chemistry)
      The local conformation of poly(Me methacrylate) (PMMA) chains at the nitrogen (N2) and water interfaces was studied by IR-visible sum-frequency generation (SFG) spectroscopy. Although SFG spectra in the C-H region for PMMA at the N2 interface have been hitherto reported, the peak assignments are not in accord with one another. Thus, we first made accurate assignments of SFG peaks using films, which had been well annealed at a temp. above the glass transition temp. for a long time, of three different deuterated PMMAs as well as normal protonated PMMA. At the N2 interface, hydrophobic functional groups such as a Me, ester Me and methylene groups were present. While the a Me group was oriented along the direction parallel to the interface, ester Me and methylene groups were oriented normal to the interface. Quant. discussion concerning the orientation of the functional groups of PMMA at the N2 interface was aided by a model calcn. Once the PMMA film contacted water, the carbonyl groups of the PMMA side chains were oriented to the water phase to form hydrogen bonds with water mols., resulting in the migration of ester Me into the internal region of the film. Concurrently, the methylene groups became randomly oriented at the water interface and/or in part migrated into the internal region. Interestingly, the Me groups still existed at the water interface oriented along the parallel direction. The outermost region of PMMA in water can consist of hydrophilic and hydrophobic domains with sub-nanometer scale. Water mols. H-bond to themselves near the hydrophobic domains, leading to the formation of an ice-like structure of water mols. However, water mols. adjacent to the hydrophilic domains H-bond with carbonyl groups.
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    40. 40
      Cimatu, K. L. A.; Ambagaspitiya, T. D.; Premadasa, U. I.; Adhikari, N. M.; Kruse, A.; Robertson, E.; Guan, S.; Rong, L.; Advincula, R.; Bythell, B. J. Polymer-Solvent Interaction and Conformational Changes at a Molecular Level: Implication to Solvent-Assisted Deformation and Aggregation at the Polymer Surface. J. Colloid Interface Sci. 2022, 616, 221– 233,  DOI: 10.1016/j.jcis.2022.02.006
      40
      Polymer-solvent interaction and conformational changes at a molecular level: Implication to solvent-assisted deformation and aggregation at the polymer surface
      Cimatu, Katherine Leslee A.; Ambagaspitiya, Tharushi D.; Premadasa, Uvinduni I.; Adhikari, Narendra M.; Kruse, Adelaide; Robertson, Emily; Guan, Shanshan; Rong, Lihan; Advincula, Rigoberto; Bythell, Benjamin J.
      Journal of Colloid and Interface Science (2022), 616 (), 221-233CODEN: JCISA5; ISSN:0021-9797. (Elsevier B.V.)
      We hypothesize that varying the chem. structure of the monomeric unit in a polymer will affect the surface structure and interfacial mol. group orientations of the polymer film leveraging its response to solvents of different chem. affinities. Poly (2-methoxy Et methacrylate) and poly (2-tertbutoxy Et methacrylate) thin films exposed to either deuterated water (D2O) or deuterated chloroform (CDCl3) were studied by sum frequency generation (SFG) spectroscopy, contact angle goniometry, and at. force microscopy (AFM) at the polymer-solvent interface, supported with mol. simulation studies. SFG spectral anal. of the polymer thin films corroborated mol. re-organization at the surface when exposed to different chem. environments. The AFM height images of the polymer surfaces were homogeneously flat under CDCl3 and showed swollen regions under D2O. Following the removal of D2O, the exposed areas have imprinted, recessed locations and exposure to CDCl3 resulted in the formation of aggregates. The chem. affinity and characteristics of the solvents played a role in conformational change at the polymer surface. It had direct implications to interfacial processes involving adsorption, permeation which eventually leads to swelling, deformation or aggregation, and possibly dissoln.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB38Xktl2msrg%253D&md5=08f2d8b0fd3ab1376cf23d65e74a6966
    41. 41
      Doughty, B.; Genix, A.-C.; Popov, I.; Li, B.; Zhao, S.; Saito, T.; Lutterman, D. A.; Sacci, R. L.; Sumpter, B. G.; Wojnarowska, Z.; Bocharova, V. Structural Correlations Tailor Conductive Properties in Polymerized Ionic Liquids. Phys. Chem. Chem. Phys. 2019, 21 (27), 14775– 14785,  DOI: 10.1039/C9CP02268F
      41
      Structural correlations tailor conductive properties in polymerized ionic liquids
      Doughty, Benjamin; Genix, Anne-Caroline; Popov, Ivan; Li, Bingrui; Zhao, Sheng; Saito, Tomonori; Lutterman, Daniel A.; Sacci, Robert L.; Sumpter, Bobby G.; Wojnarowska, Zaneta; Bocharova, Vera
      Physical Chemistry Chemical Physics (2019), 21 (27), 14775-14785CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      Polymd. ionic liqs. (PolyILs) are promising materials for applications in electrochem. devices spanning from fuel cells to capacitors and batteries. In principle, PolyILs have a competitive advantage over traditional electrolytes in being single ion conductors and thus enabling a transference no. close to unity. Despite this perceived advantage, surprisingly low room temp. ionic conductivities measured in the lab. raise an important fundamental question: how does the mol. structure mediate cond.. In this work, wide-angle X-ray scattering (WAXS), vibrational sum frequency generation (vSFG), and d. functional theory (DFT) calcns. were used to study the bulk and interfacial structure of PolyILs, while broad band dielec. spectroscopy (BDS) was used to probe corresponding dynamics and conductive properties for a series of the PolyIL samples with tunable chemistries and structures. Our results reveal that the size of the mobile anions has a tremendous impact on chain packing in PolyILs that wasn't addressed previously. Larger mobile ions tend to create a well-packed structure, while smaller ions frustrate chain packing. The magnitude of these changes and level of structural heterogeneity are shown to depend on the chem. functionality and flexibility of studied PolyILs. Furthermore, these exptl. and computational results provide new insight into the correlation between cond. and structure in PolyILs, suggesting that structural heterogeneity helps to reduce the activation energy for ionic cond. in the glassy state.
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    42. 42
      Chen, Z. Investigating Buried Polymer Interfaces Using Sum Frequency Generation Vibrational Spectroscopy. Prog. Polym. Sci. 2010, 35 (11), 1376– 1402,  DOI: 10.1016/j.progpolymsci.2010.07.003
      42
      Investigating buried polymer interfaces using sum frequency generation vibrational spectroscopy
      Chen, Zhan
      Progress in Polymer Science (2010), 35 (11), 1376-1402CODEN: PRPSB8; ISSN:0079-6700. (Elsevier Ltd.)
      A review. This paper reviews recent progress in the studies of buried polymer interfaces using sum frequency generation (SFG) vibrational spectroscopy. Both buried solid/liq. and solid/solid interfaces involving polymeric materials are discussed. SFG studies of polymer/water interfaces show that different polymers exhibit varied surface restructuring behavior in water, indicating the importance of probing polymer/water interfaces in situ. SFG has also been applied to the investigation of interfaces between polymers and other liqs. It has been found that mol. interactions at such polymer/liq. interfaces dictate interfacial polymer structures. The mol. structures of silane mols., which are widely used as adhesion promoters, have been investigated using SFG at buried polymer/silane and polymer/polymer interfaces, providing mol.-level understanding of polymer adhesion promotion. The mol. structures of polymer/solid interfaces have been examd. using SFG with several different exptl. geometries. These results have provided mol.-level information about polymer friction, adhesion, interfacial chem. reactions, interfacial electronic properties, and the structure of layer-by-layer deposited polymers. Such research has demonstrated that SFG is a powerful tool to probe buried interfaces involving polymeric materials, which are difficult to study by conventional surface sensitive anal. techniques.
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    43. 43
      Wang, J.; Paszti, Z.; Even, M. A.; Chen, Z. Measuring Polymer Surface Ordering Differences in Air and Water by Sum Frequency Generation Vibrational Spectroscopy. J. Am. Chem. Soc. 2002, 124 (24), 7016– 7023,  DOI: 10.1021/ja012387r
      43
      Measuring Polymer Surface Ordering Differences in Air and Water by Sum Frequency Generation Vibrational Spectroscopy
      Wang, Jie; Paszti, Zoltan; Even, Mark A.; Chen, Zhan
      Journal of the American Chemical Society (2002), 124 (24), 7016-7023CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Mol. structures of poly(Bu methacrylate) (PBMA) at the PBMA/air and PBMA/water interfaces have been studied by sum frequency generation (SFG) vibrational spectroscopy. PBMA surfaces in both air and water are dominated by the Me groups of the ester side chains. The av. orientation and orientation distribution of these Me groups at the PBMA/air and PBMA/water interfaces are different, indicating that surface restructuring occurs when the PBMA sample contacts water. Anal. shows that the orientation distribution of side chain Me groups on the PBMA surface is narrower in water than that in air, indicating that the PBMA surface can be more ordered in water. To our knowledge, this is the first time that quant. comparisons between mol. surface structures of polymers in air and in water have been made. Two assumptions on the orientation distribution function, including a Gaussian distribution and a formula based on the max. entropy approach, are used in the anal. It has been found that the orientation angle distribution function deduced by the Gaussian distribution and the max. entropy distribution are quite similar, showing that the Gaussian distribution is a good approxn. for the angle distribution. The effect of exptl. error on the deduced orientational distribution is also discussed.
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    44. 44
      Lambert, A. G.; Davies, P. B.; Neivandt, D. J. Implementing the Theory of Sum Frequency Generation Vibrational Spectroscopy: A Tutorial Review. Appl. Spectrosc. Rev. 2005, 40 (2), 103– 145,  DOI: 10.1081/ASR-200038326
      44
      Implementing the theory of sum frequency generation vibrational spectroscopy: a tutorial review
      Lambert, Alex G.; Davies, Paul B.; Neivandt, David J.
      Applied Spectroscopy Reviews (2005), 40 (2), 103-145CODEN: APSRBB; ISSN:0570-4928. (Taylor & Francis, Inc.)
      A review. The interfacial regions between bulk media, although often comprising only a fraction of the material present, are frequently the site of reactions and phenomena that dominate the macroscopic properties of the entire system. Spectroscopic investigations of such interfaces are often hampered by the lack of surface specificity of most available techniques. Sum frequency generation vibrational spectroscopy (SFS) is a non-linear optical technique which provides vibrational spectra of mols. solely at interfaces. The spectra may be analyzed to provide the polar orientation, mol. conformation, and av. tilt angle of the adsorbate to the surface normal. This article is aimed at newcomers to the field of SFS, and via a tutorial approach will present and develop the general sum frequency equations and then demonstrate how the fundamental theory elucidates the important exptl. properties of SFS.
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    45. 45
      Wang, H.-F.; Gan, W.; Lu, R.; Rao, Y.; Wu, B.-H. Quantitative Spectral and Orientational Analysis in Surface Sum Frequency Generation Vibrational Spectroscopy (SFG-VS). Int. Rev. Phys. Chem. 2005, 24 (2), 191– 256,  DOI: 10.1080/01442350500225894
      45
      Quantitative spectral and orientational analysis in surface sum frequency generation vibrational spectroscopy (SFG-VS)
      Wang, Hong-Fei; Gan, Wei; Lu, Rong; Rao, Yi; Wu, Bao-Hua
      International Reviews in Physical Chemistry (2005), 24 (2), 191-256CODEN: IRPCDL; ISSN:0144-235X. (Taylor & Francis Ltd.)
      A review. Sum frequency generation vibrational spectroscopy (SFG-VS) was proven to be a uniquely effective spectroscopic technique in the study of mol. structure and conformations, as well as the dynamics of mol. interfaces. The ability to apply SFG-VS to complex mol. interfaces was limited by the ability to abstr. quant. information from SFG-VS expts. The authors try to make assessments of the limitations, issues and techniques as well as methodols. in quant. orientational and spectral anal. with SFG-VS. Based on these assessments, the authors also try to summarize recent developments in methodologies on quant. orientational and spectral anal. in SFG-VS, and their applications to detailed anal. of SFG-VS data of various vapor/neat liq. interfaces. A rigorous formulation of the polarization null angle (PNA) method is given for accurate detn. of the orientational parameter D = 〈cos θ 〉/〈cos3 θ〉, and comparison between the PNA method with the commonly used polarization intensity ratio (PIR) method is discussed. The polarization and incident angle dependencies of the SFG-VS intensity are reviewed, in the light of how exptl. arrangements can be optimized to effectively abstr. crucial information from the SFG-VS expts. The values and models of the local field factors in the mol. layers are discussed. To examine the validity and limitations of the bond polarizability deriv. model, the general expressions for mol. hyperpolarizability tensors and their expression with the bond polarizability deriv. model for C3v, C2v and C∞v mol. groups are given in the 2 appendixes. The bond polarizability deriv. model can quant. describe many aspects of the intensities obsd. in the SFG-VS spectrum of the vapor/neat liq. interfaces in different polarizations. Using the polarization anal. in SFG-VS, polarization selection rules or guidelines are developed for assignment of the SFG-VS spectrum. Using the selection rules, SFG-VS spectra of vapor/diol, and vapor/n-normal alc. (n ∼ 1-8) interfaces are assigned, and some of the ambiguity and confusion, as well as their implications in previous IR and Raman assignment, are duly discussed. The ability to assign a SFG-VS spectrum using the polarization selection rules makes SFG-VS not only an effective and useful vibrational spectroscopy technique for interface studies, but also a complementary vibrational spectroscopy method in general condensed phase studies. These developments will put quant. orientational and spectral anal. in SFG-VS on a more solid foundation. The formulations, concepts and issues discussed in this review are expected to find broad applications for studies on mol. interfaces in the future.
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    46. 46
      Wang, H.-F.; Velarde, L.; Gan, W.; Fu, L. Quantitative Sum-Frequency Generation Vibrational Spectroscopy of Molecular Surfaces and Interfaces: Lineshape, Polarization, and Orientation. Annu. Rev. Phys. Chem. 2015, 66 (1), 189– 216,  DOI: 10.1146/annurev-physchem-040214-121322
      46
      Quantitative sum-frequency generation vibrational spectroscopy of molecular surfaces and interfaces: lineshape, polarization, and orientation
      Wang, Hong-Fei; Velarde, Luis; Gan, Wei; Fu, Li
      Annual Review of Physical Chemistry (2015), 66 (), 189-216CODEN: ARPLAP; ISSN:0066-426X. (Annual Reviews)
      Sum-frequency generation vibrational spectroscopy (SFG-VS) can provide detailed information and understanding of the mol. compn., interactions, and orientational and conformational structure of surfaces and interfaces through quant. measurement and anal. In this review, we present the current status of and discuss important recent developments in the measurement of intrinsic SFG spectral lineshapes and formulations for polarization measurements and orientational anal. of SFG-VS spectra. The focus of this review is to present a coherent description of SFG-VS and discuss the main concepts and issues that can help advance this technique as a quant. anal. research tool for revealing the chem. and physics of complex mol. surfaces and interfaces.
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    47. 47
      Li, T.; Mao, D.; Petrone, N. W.; Grassi, R.; Hu, H.; Ding, Y.; Huang, Z.; Lo, G.-Q.; Hone, J. C.; Low, T.; Wong, C. W.; Gu, T. Spatially Controlled Electrostatic Doping in Graphene P-i-n Junction for Hybrid Silicon Photodiode. npj 2D Mater. Appl. 2018, 2 (1), 1– 8,  DOI: 10.1038/s41699-018-0080-4
      There is no corresponding record for this reference.
    48. 48
      Aftab, S.; Hegazy, H. H.; Iqbal, M. Z.; Iqbal, M. W.; Nazir, G.; Hussain, S. Recent Advances in Dynamic Homojunction PIN Diodes Based on 2D Materials. Advanced Materials Interfaces 2023, 10 (6), 2201937,  DOI: 10.1002/admi.202201937
      There is no corresponding record for this reference.
    49. 49
      Kawai, S.; Saito, S.; Osumi, S.; Yamaguchi, S.; Foster, A. S.; Spijker, P.; Meyer, E. Atomically Controlled Substitutional Boron-Doping of Graphene Nanoribbons. Nat. Commun. 2015, 6 (1), 8098,  DOI: 10.1038/ncomms9098
      49
      Atomically controlled substitutional boron-doping of graphene nanoribbons
      Kawai, Shigeki; Saito, Shohei; Osumi, Shinichiro; Yamaguchi, Shigehiro; Foster, Adam S.; Spijker, Peter; Meyer, Ernst
      Nature Communications (2015), 6 (), 8098CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
      Boron is a unique element in terms of electron deficiency and Lewis acidity. Incorporation of boron atoms into an arom. carbon framework offers a wide variety of functionality. However, the intrinsic instability of organoboron compds. against moisture and oxygen has delayed the development. Here, we present boron-doped graphene nanoribbons (B-GNRs) of widths of N=7, 14 and 21 by on-surface chem. reactions with an employed organoboron precursor. The location of the boron dopant is well defined in the center of the B-GNR, corresponding to 4.8 atom%, as programmed. The chem. reactivity of B-GNRs is probed by the adsorption of nitric oxide (NO), which is most effectively trapped by the boron sites, demonstrating the Lewis acid character. Structural properties and the chem. nature of the NO-reacted B-GNR are detd. by a combination of scanning tunnelling microscopy, high-resoln. at. force microscopy with a CO tip, and d. functional and classical computations.
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    50. 50
      Huh, S.; Park, J.; Kim, K. S.; Hong, B. H.; Kim, S. B. Selective N-Type Doping of Graphene by Photo-Patterned Gold Nanoparticles. ACS Nano 2011, 5 (5), 3639– 3644,  DOI: 10.1021/nn1035203
      50
      Selective n-Type Doping of Graphene by Photo-patterned Gold Nanoparticles
      Huh, Sung; Park, Jaesung; Kim, Kwang S.; Hong, Byung Hee; Kim, Seung Bin
      ACS Nano (2011), 5 (5), 3639-3644CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Selective n-type doping of graphene is developed by utilizing patternable gold nanoparticles functionalized with photoreactive cinnamate moieties. The gold nanoparticles can be regularly patterned on the graphene by UV-induced crosslinking of cinnamate, which provides a convenient method to control the optical and elec. properties of graphene site-specifically. The strong n-type doping of graphene covered with the patterned gold nanoparticles was confirmed by Raman, XPS, and electron transport measurements. We believe that our method would find numerous applications in the area of graphene-based optoelectronics including light-emitting devices, solar cells, and optical sensors.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXkvVSntro%253D&md5=e2a290e7aef4ce92c24312457cac7992
    51. 51
      Lee, J.; Novoselov, K. S.; Shin, H. S. Interaction between Metal and Graphene: Dependence on the Layer Number of Graphene. ACS Nano 2011, 5 (1), 608– 612,  DOI: 10.1021/nn103004c
      51
      Interaction between Metal and Graphene: Dependence on the Layer Number of Graphene
      Lee, Jisook; Novoselov, Konstantin S.; Shin, Hyeon Suk
      ACS Nano (2011), 5 (1), 608-612CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      The interaction between graphene and metal was studied by studying the G band splitting in surface-enhanced Raman scattering (SERS) spectra of single-, bi-, and trilayer graphene. The Ag deposition on graphene induced large enhancement of the Raman signal of graphene, indicating SERS of graphene. In particular, the G band was split into two distinct peaks in the SERS spectrum of graphene. The extent of the G band splitting was 13.0 cm-1 for single-layer, 9.6 cm-1 for bilayer, and 9.4 cm-1 for trilayer graphene, whereas the G band in the SERS spectrum of a thick multilayer was not split. The av. SERS enhancement factor of the G band was 24 for single-layer, 15 for bilayer, and 10 for trilayer graphene. There is a correlation between SERS enhancement factor and the extent of the G band splitting, and the strongest interaction occurs between Ag and single-layer graphene. Also, the Ag deposition on graphene can induce doping of graphene. The intensity ratio of 2-dimensional and G bands (I2D/IG) decreased after Ag deposition on graphene, indicating doping of graphene. From changes in positions of G and 2-dimensional bands after the metal deposition on graphene, Ag deposition induced n-doping of graphene, whereas Au deposition induced p-doping.
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    52. 52
      Lin, L.; Chowdhury, A. U.; Ma, Y.-Z.; Sacci, R. L.; Katsaras, J.; Hong, K.; Collier, C. P.; Carrillo, J.-M. Y.; Doughty, B. Ion Pairing Mediates Molecular Organization Across Liquid/Liquid Interfaces. ACS Appl. Mater. Interfaces 2021, 13 (28), 33734– 33743,  DOI: 10.1021/acsami.1c09763
      52
      Ion Pairing Mediates Molecular Organization Across Liquid/Liquid Interfaces
      Lin, Lu; Chowdhury, Azhad U.; Ma, Ying-Zhong; Sacci, Robert L.; Katsaras, John; Hong, Kunlun; Collier, C. Patrick; Carrillo, Jan-Michael Y.; Doughty, Benjamin
      ACS Applied Materials & Interfaces (2021), 13 (28), 33734-33743CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Liq./liq. interfaces play a central role in scientific fields ranging from nanomaterial synthesis and soft matter electronics to nuclear waste remediation and chem. sepns. This diversity of functions arises from an interface's ability to respond to changing conditions in its neighboring bulk phases. Understanding what drives this interfacial flexibility can provide novel avenues for designing new functional interfaces. However, limiting this progress is an inadequate understanding of the subtle intermol. and interphase interactions taking place at the mol. level. Here, we use surface-specific vibrational sum frequency generation spectroscopy combined with atomistic mol. dynamics simulations to investigate the self-assembly and structure of model ionic oligomers consisting of an oligodimethylsiloxane (ODMS) tail covalently attached to a pos. charged Me imidazolium (MIM+) head group at buried oil/aq. interfaces. We show how the presence of seemingly innocuous salts can impart dramatic changes to the ODMS tail conformations in the oil phase via specific ion effects and ion-pairing interactions taking place in the aq. phase. These specific ion interactions are shown to drive enhanced amphiphile adsorption, induce morphol. changes, and disrupt emergent hydrogen-bonding structures at the interface. Tuning these interactions allows for independent control over the oligomer structure in the oil phase vs. interfacial population changes and represents key mechanistic insight that is needed to control chem. reactions at liq./liq. interfaces.
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    53. 53
      Premadasa, U. I.; Dong, D.; Stamberga, D.; Custelcean, R.; Roy, S.; Ma, Y.-Z.; Bocharova, V.; Bryantsev, V. S.; Doughty, B. Chemical Feedback in the Self-Assembly and Function of Air-Liquid Interfaces: Insight into the Bottlenecks of CO2 Direct Air Capture. ACS Appl. Mater. Interfaces 2023, 15 (15), 19634– 19645,  DOI: 10.1021/acsami.3c00719
      53
      Chemical Feedback in the Self-Assembly and Function of Air-Liquid Interfaces: Insight into the Bottlenecks of CO2 Direct Air Capture
      Premadasa, Uvinduni I.; Dong, Dengpan; Stamberga, Diana; Custelcean, Radu; Roy, Santanu; Ma, Ying-Zhong; Bocharova, Vera; Bryantsev, Vyacheslav S.; Doughty, Benjamin
      ACS Applied Materials & Interfaces (2023), 15 (15), 19634-19645CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      As fossil fuels remain a major source of energy throughout the world, developing efficient neg. emission technologies, such as direct air capture (DAC), which remove carbon dioxide (CO2) from the air, becomes crit. for mitigating climate change. Although all DAC processes involve CO2 transport from air into a sorbent/solvent, through an air-solid or air-liq. interface, the fundamental roles the interfaces play in DAC remain poorly understood. Herein, we study the interfacial behavior of amino acid (AA) solvents used in DAC through a combination of vibrational sum frequency generation spectroscopy and mol. dynamics simulations. This study revealed that the absorption of atm. CO2 has antagonistic effects on subsequent capture events that are driven by changes in bulk pH and specific ion effects that feedback on surface organization and interactions. Among the three AAs (leucine, valine, and phenylalanine) studied, we identify and sep. behaviors from CO2 loading, chem. changes, variations in pH, and specific ion effects that tune structural and chem. degrees of freedom at the air-aq. interface. The fundamental mechanistic findings described here are anticipated to enable new approaches to DAC based on exploiting interfaces as a tool to address climate change.
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    54. 54
      Premadasa, U. I.; Bocharova, V.; Lin, L.; Genix, A.-C.; Heller, W. T.; Sacci, R. L.; Ma, Y.-Z.; Thiele, N. A.; Doughty, B. Tracking Molecular Transport Across Oil/Aqueous Interfaces: Insight into “Antagonistic” Binding in Solvent Extraction. J. Phys. Chem. B 2023, 127 (21), 4886– 4895,  DOI: 10.1021/acs.jpcb.3c00386
      There is no corresponding record for this reference.
    55. 55
      Ohno, P. E.; Saslow, S. A.; Wang, H.; Geiger, F. M.; Eisenthal, K. B. Phase-Referenced Nonlinear Spectroscopy of the α-Quartz/Water Interface. Nat. Commun. 2016, 7 (1), 13587,  DOI: 10.1038/ncomms13587
      55
      Phase-referenced nonlinear spectroscopy of the α-quartz/water interface
      Ohno, Paul E.; Saslow, Sarah A.; Wang, Hong-fei; Geiger, Franz M.; Eisenthal, Kenneth B.
      Nature Communications (2016), 7 (), 13587CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
      Probing the polarization of water mols. at charged interfaces by second harmonic generation spectroscopy has been heretofore limited to isotropic materials. Here we report non-resonant nonlinear optical measurements at the interface of anisotropic z-cut α-quartz and water under conditions of dynamically changing ionic strength and bulk soln. pH. We find that the product of the third-order susceptibility and the interfacial potential, χ(3) × Φ(0), is given by (χ1(3)-iχ2(3)) × Φ(0), and that the interference between this product and the second-order susceptibility of bulk quartz depends on the rotation angle of α-quartz around the z axis. Our expts. show that this newly identified term, iχ(3) × Φ(0), which is out of phase from the surface terms, is of bulk origin. The possibility of internally phase referencing the interfacial response for the interfacial orientation anal. of species or materials in contact with α-quartz is discussed along with the implications for conditions of resonance enhancement.
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  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acscentsci.4c00704.

    • Synthetic methods and characterization of the monomers and polymers; ellipsometry data; supporting SFG spectra, fitting results, details on orientational analysis; supplementary KPFM and AFM profiles, DSC curves, and optical images (PDF)


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