Substrate-Selective Adhesion of Metal Nanoparticles to Graphene Devices

Nanostructured electronic devices, such as those based on graphene, are typically grown on top of the insulator SiO2. Their exposure to a flux of small size-selected silver nanoparticles has revealed remarkably selective adhesion: the graphene channel can be made fully metallized, while the insulating substrate remains coverage-free. This conspicuous contrast derives from the low binding energy between the metal nanoparticles and a contaminant-free passivated silica surface. In addition to providing physical insight into nanoparticle adhesion, this effect may be of value in applications involving deposition of metallic layers on device working surfaces: it eliminates the need for masking the insulating region and the associated extensive and potentially deleterious pre- and postprocessing.


S-I. Device fabrication
Graphene FET devices were fabricated in a cleanroom environment at the Aerospace Corporation. Square 1 cm × 1 cm dies were cut from a wafer of CVD graphene grown on a Si/SiO2 substrate. Each die was then patterned with a 5 × 5 array of four-probe graphene FETs via a multistep lithography process. The rectangular graphene channel was defined by electron beam lithography (EBL) and etched out of the graphene layer by using an argon plasma. Then an additional EBL and electron beam physical vapor deposition (EBPVD) process was performed to deposit four Ti/Au (10nm/50 nm) contacts onto the channel surface, completing the device. An image of one FET is shown in Figure S1.  S-4 S-III. Nanoparticle deposition on graphene patches Figure S3. Silver nanoparticles cover electrically isolated islands of graphene but not the surrounding expanse of SiO2. There also is no evidence of nanoparticle aggregation along the graphene step edges, and hence of any significant surface diffusion. S-5

S-IV. Nanoparticle deposition on mica
A disc of mica (Ted Pella, optically flat grade V1) was cleaved in air and mounted adjacent to a wafer of CVD graphene in the same manner as the samples described in the main text. Analogously to the graphene substrate, the mica surface possesses an organized crystal structure and provides a very flat surface upon which to deposit nanoparticles. S1, S2 However, analogously to silica, it is electrically insulating. Imaging shows that the mica and the graphene substrates become similarly covered. Figure S4. Insulating mica samples show a similar degree of nanoparticle coverage as the adjacent CVD graphene substrates.

S-V. Contact-mode nanomanipulation
Nanomanipulation experiments were conducted using Ir/Ti coated conductive silicon tips (Oxford Instruments ASYELEC.01-R2). The procedure is first to obtain a small (~1 μm 2 ) fieldof-view image of the deposited nanoparticles near a graphene/SiO2 step edge (see Figure S4 top left). This first image is acquired in the non-contact attractive regime, so as not to disturb the particles, as described in the main text. Once this image is obtained, the system allows for seamless transition to contact-mode imaging where custom tip deflection settings and paths can be preset by using the collected image as a reference (see the path overlay in Figure S4 top right). The paths are then traced out, in order, while the tip is in contact with the sample surface. For all contact traces the tip speed is set to the slowest possible scan rate of 5 nm/s, and the tip lifts off the surface when relocating from the end of one trace to the beginning of the next. After all contact traces are completed, the image area is finally rescanned in the non-contact mode to reveal the result of the manipulations. Figure S4 depicts a representative manipulation cycle and displays the range of observed outcomes.
Trace 1 shows the move of a nanoparticle off the graphene onto the cleaned SiO2 surface.
Trace 2 demonstrates a frequent occurrence for a nanoparticle that had already been moved to the SiO2 during a previous manipulation. This trace sought simply to move it along the oxide surface, but the post-manipulation image finds that the particle has been "erased" from the region ( Figure S4 bottom). What happened is that the nanoparticle desorbed from the substrate and attached itself to the Ir/Ti coated tip. Such an outcome also has been observed while attempting to move nanoparticles from graphene to SiO2 (akin to trace 1). Finally, trace 3 relocated a cluster along the graphene toward the SiO2 interface. A close inspection of the result reveals not one but two nanoparticles at the final location (highlighted by the blue box). Thus, it appears that the above tip-adsorbed nanoparticle has been released onto the graphene surface.
The attachment of nanoparticles to the tip was observed only when moving them along the clean SiO2 substrate, and nanoparticle were never removed from the graphene. Additionally, nanoparticle redeposition was only observed onto the graphene surface. This suggests that while S-7 the nanoparticle interaction with the tip is stronger than that with the oxide surface, the nanoparticle-graphene interaction is stronger still. S-8

S-VI. Computational details
The calculations are carried out within the density functional theory (DFT) framework using the Vienna ab initio simulation package (VASP) S3 which employs periodic boundary conditions and plane-wave basis sets. The projector augmented wave (PAW) S4 method is used to describe the electron-ion interactions, and the Perdew-Burke-Ernzerhof (PBE) S5 functional is applied to account for the electronic exchange-correlation interactions. A uniform 1×1×1 Monkhorst-Pack kpoint sampling S6 and a plane-wave energy cutoff of 400 eV are utilized. The van der Waals interactions are described using the Grimme DFT-D3 method with the Becke-Johnson damping. S7 The geometry optimization is considered converged when ion forces become less than 10 −3 eV·Å −1 . The SiO2 surface contains silanol groups (Si-OH) forming a zigzag hydrogen bonded network. A 20 Å vacuum layer is added to the surface normal in all systems to avoid spurious interactions between the periodic images.