Ultrafast Transient Terahertz Conductivity of Monolayer MoS2 and WSe2 Grown by Chemical Vapor DepositionClick to copy article linkArticle link copied!
- Callum J. Docherty
- Patrick Parkinson
- Hannah J. Joyce
- Ming-Hui Chiu
- Chang-Hsiao Chen
- Ming-Yang Lee
- Lain-Jong Li
- Laura M. Herz
- Michael B. Johnston
Abstract
We have measured ultrafast charge carrier dynamics in monolayers and trilayers of the transition metal dichalcogenides MoS2 and WSe2 using a combination of time-resolved photoluminescence and terahertz spectroscopy. We recorded a photoconductivity and photoluminescence response time of just 350 fs from CVD-grown monolayer MoS2, and 1 ps from trilayer MoS2 and monolayer WSe2. Our results indicate the potential of these materials as high-speed optoelectronic materials.
Figure 1
Figure 1. Characterization of monolayer MoS2 (a–d), trilayer MoS2 (e–h), and monolayer WSe2 samples. (a), (e), (i) Schematic representation of the samples. (b), (f), (j) Raman spectroscopy of the samples. The splitting between the E2G1 and A1G peaks is indicative of layer number. (c), (g), (k) AFM images of the samples. Height profiles are overlaid at the position they were taken. The step heights again confirm the layer number identification. (d), (h), (l) Optical transmission spectrum of the samples. Absorption features associated with the “A” and “B” excitons are labeled.
Sample Preparation and Characterization
Results
Figure 2
Figure 2. Photodynamics of monolayer MoS2 measured by optical pump–THz probe spectroscopy and PL upconversion spectroscopy. (a) The blue circles show the normalized THz photoconductivity, ΔσTHz, of monolayer MoS2 as a function of time after photoexcitation by 3.1 eV photons (absorbed fluence ∼7 × 1013 cm–2/pulse). The black crosses show the normalized PL emitted at 1.86 eV as a function of time after photoexcitation by 3 eV photons (absorbed fluence ∼6.9 × 1012 cm–2/pulse). Solid lines are biexponential fits to the data. (b) Similar ΔσTHz (blue circles) and PL (black crosses) measurements following photoexcitation on resonance with the direct gap excitons (1.9 eV photons with absorbed fluence ∼1.2 × 1014 cm–2 for ΔσTHz, and 2.1 eV photons with absorbed fluence ∼3.5 × 1012 cm–2 for PL).
Figure 3
Figure 3. Normalized photodynamics of monolayer WSe2 measured by optical pump-THz probe spectroscopy. The blue crosses show the THz photoconductivity of WSe2, ΔσTHz, as a function of time after photoexcitation by 35 fs pulses of above-gap photons with energy of 3.1 eV. The absorbed photon fluence was ∼4.2 × 1013 cm–2/pulse. The red squares show ΔσTHz of WSe2 photoexcited resonantly with the “A” exciton at a photon energy of 1.65 eV. The fluence absorbed in the WSe2 layer was ∼3.3 × 1013 cm–2/pulse. Solid lines are biexponential fits to the data.
Figure 4
Figure 4. Comparison of THz photoconductivity, ΔσTHz, in trilayer (green crosses) and monolayer (blue squares) MoS2 as a function of time after photoexcitation by 175 μJ cm–2 35 fs pulses of 3.1 eV photons. The absorbed fluence of photons were ∼1.4 × 1014 cm–2/pulse in the trilayer sample and ∼7 × 1013 cm–2/pulse in the monolayer sample. Solid lines show biexponential fits to the data. Fit parameters are listed in Supporting Information Table 1. Additional reflections due to the thinness of the substrate have been removed from the trilayer data for clarity (see Supporting Information).
Figure 5
Figure 5. THz complex photoconductivity spectra, Δσ(ν). (a) Monolayer MoS2 1.5 ps after photoexcitation with 3 eV (squares) and 1.9 eV (crosses) photons. A low energy resonance at ∼18 meV (4.5 THz) is apparent. (b) Trilayer MoS2, 1.5 ps after photoexcitation by 1.9 eV photons. (c) Monolayer WSe2, 1 ps after photoexcitation on resonance with the WSe2 “A” exciton at 1.65 eV. Solid lines show possible fits to the data, detailed in the Supporting Information.
Conclusion
Methods
Sample Preparation
Photoluminescence Spectroscopy
THz Spectroscopy
Supporting Information
detailing the pump fluence and pump energy dependence of the THz spectroscopy measurements, as well as information above removing reflections from the trilayer THz photoconductivity. This material is available free of charge via the Internet at http://pubs.acs.org.
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.
Acknowledgment
We would like to thank A. Prabakaran and N. Grobert for performing Raman mapping measurments on the samples after the THz measurments were completed. The authors would like to thank the EPSRC (UK), Academia Sinica (IAMS and Nano program) and National Science Council Taiwan (NSC-99-2112-M-001-021-MY3) for financial support.
References
This article references 59 other publications.
- 1Smith, F. W.; Le, H. Q.; Diadiuk, V.; Hollis, M. A.; Calawa, A. R.; Gupta, S.; Frankel, M.; Dykaar, D. R.; Mourou, G. A.; Hsiang, T. Y. Picosecond GaAs-based Photoconductive Optoelectronic Detectors Appl. Phys. Lett. 1989, 54, 890– 892Google ScholarThere is no corresponding record for this reference.
- 2Nolte, D. D. Semi-insulating Semiconductor Heterostructures: Optoelectronic Properties and Applications J. Appl. Phys. 1999, 85, 6259– 6289Google Scholar2https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK1MXisFCiur8%253D&md5=faaad20367c407efdfda885b9522c0cbSemi-insulating semiconductor heterostructures: Optoelectronic properties and applicationsNolte, David D.Journal of Applied Physics (1999), 85 (9), 6259-6289CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)This review with 279 refs. covers a spectrum of optoelectronic properties of and uses for semi-insulating semiconductor heterostructures and thin films, including epilayers and quantum wells. Compensation by doping, implantation, and nonstoichiometric growth are described in terms of the properties of point defects and Fermi level stabilization and pinning. The principal optical and optoelectronic properties of semi-insulating epilayers and heterostructures, such as excitonic electroabsorption of quantum-confined excitons, are described, in addn. to optical absorption by metallic or semimetallic ppts. in these layers. Low-temp. grown quantum wells that have an As-rich nonstoichiometry and a supersatd. concn. of grown-in vacancies are discussed. These heterostructures experience transient enhanced diffusion and superlattice disordering. The review discusses the performance of optoelectronic heterostructures and microcavities that contain semi-insulating layers, such as buried heterostructure stripe lasers, vertical cavity surface emitting lasers, and optical electroabsorption modulators. Short time-scale applications arise from the ultrashort carrier lifetimes in semi-insulating materials, such as in photoconductors for terahertz generation, and in saturable absorbers for mode-locking solid-state lasers. This review also comprehensively describes the properties and applications of photorefractive heterostructures. The low dark-carrier concns. of semi-insulating heterostructures make these materials highly sensitive as dynamic holog. thin films that are useful for adaptive optics applications. The high mobilities of free carriers in photorefractive heterostructures produce fast dielec. relaxation rates that allow light-induced space-charge gratings to adapt to rapidly varying optical fringe patterns, canceling out environmental noise during interferometric detection in laser-based ultrasound, and in optical coherence tomog. They are also the functional layers in high-sensitivity dynamic holog. materials that replace static holograms in Fourier imaging systems and in exptl. Tbit/s optical systems. Semi-insulating heterostructures and their applications have attained a degree of maturity, but many crit. materials science issues remain unexplored.
- 3Krotkus, A. Semiconductors for Terahertz Photonics Applications J. Phys. D: Appl. Phys. 2010, 43, 273001Google Scholar3https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXpsVOltrw%253D&md5=70a0b88ad1d1dada87c45ffecdd8d41cSemiconductors for terahertz photonics applicationsKrotkus, ArunasJournal of Physics D: Applied Physics (2010), 43 (27), 273001/1-273001/21CODEN: JPAPBE; ISSN:0022-3727. (Institute of Physics Publishing)A review. Generation and measurement of ultrashort, subpicosecond pulses of electromagnetic radiation with their characteristic Fourier spectra that reach far into terahertz (THz) frequency range has recently become a versatile tool of far-IR spectroscopy and imaging. This technique, THz time-domain spectroscopy, in addn. to a femtosecond pulse laser, requires semiconductor components manufd. from materials with a short photoexcited carrier lifetime, high carrier mobility and large dark resistivity. Here we will review the most important developments in the field of investigation of such materials. The main characteristics of low-temp.-grown or ion-implanted GaAs and semiconducting compds. sensitive in the wavelength ranges around 1 μm and 1.5 μm will be surveyed. The second part of the paper is devoted to the effect of surface emission of THz transients from semiconductors illuminated by femtosecond laser pulses. The main phys. mechanisms leading to this emission as well as their manifestation in various crystals will be described.
- 4Loka, H. S.; Benjamin, S. D.; Smith, P. W. E. Optical Characterization of Low-temperature-grown GaAs for Ultrafast All-optical Switching Devices IEEE J. Quantum Electron. 1998, 34, 1426– 1437Google ScholarThere is no corresponding record for this reference.
- 5Shen, Y. C.; Upadhya, P. C.; Linfield, E. H.; Beere, H. E.; Davies, A. G. Ultrabroadband Terahertz Radiation from Low-temperature-grown GaAs Photoconductive Emitters Appl. Phys. Lett. 2003, 83, 3117– 3119Google Scholar5https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3sXotVarurs%253D&md5=39bee2eb97d6cb38fc9e2832bb97885cUltrabroadband terahertz radiation from low-temperature-grown GaAs photoconductive emittersShen, Y. C.; Upadhya, P. C.; Linfield, E. H.; Beere, H. E.; Davies, A. G.Applied Physics Letters (2003), 83 (15), 3117-3119CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)THz radiation was generated with a biased and asym. excited low-temp.-grown GaAs photoconductive emitter, and characterized with a 20-μm-thick ZnTe crystal using free-space electrooptic sampling. Using a backward collection scheme, THz radiation with frequency components >30 THz were obtained, the highest ever obsd. for photoconductive emitters. Spectra over ν = 0.3-20 THz are presented, demonstrating the use of this source for ultrabroadband THz spectroscopy.
- 6Ferguson, B.; Zhang, X.-C. Materials for Terahertz Science and Technology Nat. Mater. 2002, 1, 26– 33Google Scholar6https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD38XntlWlsrc%253D&md5=39a00a51aea9eed67983a713682676a2Materials for terahertz science and technologyFerguson, Bradley; Zhang, Xi-ChengNature Materials (2002), 1 (1), 26-33CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)A review. Terahertz spectroscopy systems use far-IR radiation to ext. mol. spectral information in an otherwise inaccessible portion of the electromagnetic spectrum. Materials research is an essential component of modem terahertz systems: novel, higher-power terahertz sources rely heavily on new materials such as quantum cascade structures. At the same time, terahertz spectroscopy and imaging provide a powerful tool for the characterization of a broad range of materials, including semiconductors and biomols.
- 7Castro-Camus, E.; Lloyd-Hughes, J.; Fu, L.; Tan, H. H.; Jagadish, C.; Johnston, M. B. An Ion-implanted InP Receiver for Polarization Resolved Terahertz Spectroscopy Opt. Express 2007, 15, 7047– 7057Google ScholarThere is no corresponding record for this reference.
- 8Mangeney, J. THz Photoconductive Antennas Made from Ion-bombarded Semiconductors J. Infrared, Millimeter, Terahertz Waves 2012, 33, 455– 473Google ScholarThere is no corresponding record for this reference.
- 9Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Two-dimensional Atomic Crystals Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 10451– 10453Google Scholar9https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2MXntVSit7g%253D&md5=1ce9e5f5eb0f7b9abb033d4a690d49c3Two-dimensional atomic crystalsNovoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K.Proceedings of the National Academy of Sciences of the United States of America (2005), 102 (30), 10451-10453CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)The authors report free-standing at. crystals that are strictly 2-dimensional and can be viewed as individual at. planes pulled out of bulk crystals or as unrolled single-wall nanotubes. By using micromech. cleavage, the authors prepd. and studied a variety of 2-dimensional crystals including single layers of boron nitride, graphite, several dichalcogenides, and complex oxides. These atomically thin sheets (essentially gigantic 2-dimensional mols. unprotected from the immediate environment) are stable under ambient conditions, exhibit high crystal quality, and are continuous on a macroscopic scale.
- 10Friend, R. H.; Yoffe, A. D. Electronic-properties of Intercalation Complexes of the Transition-metal Dichalcogenides Adv. Phys. 1987, 36, 1– 94Google ScholarThere is no corresponding record for this reference.
- 11Wang, Q. H.; Kalantar-zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Electronics and Optoelectronics of Two-dimensional Transition Metal Dichalcogenides Nat. Nanotechnol. 2012, 7, 699– 712Google Scholar11https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38Xhs1ajtr7P&md5=4e45d586c6ac7b0676a461f61a53db68Electronics and optoelectronics of two-dimensional transition metal dichalcogenidesWang, Qing Hua; Kalantar-Zadeh, Kourosh; Kis, Andras; Coleman, Jonathan N.; Strano, Michael S.Nature Nanotechnology (2012), 7 (11), 699-712CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)A review. The remarkable properties of graphene have renewed interest in inorg., two-dimensional materials with unique electronic and optical attributes. Transition metal dichalcogenides (TMDCs) are layered materials with strong in-plane bonding and weak out-of-plane interactions enabling exfoliation into two-dimensional layers of single unit cell thickness. Although TMDCs were studied for decades, recent advances in nanoscale materials characterization and device fabrication have opened up new opportunities for two-dimensional layers of thin TMDCs in nanoelectronics and optoelectronics. TMDCs such as MoS2, MoSe2, WS2 and WSe2 have sizable bandgaps that change from indirect to direct in single layers, allowing applications such as transistors, photodetectors and electroluminescent devices. The authors review the historical development of TMDCs, methods for prepg. atomically thin layers, their electronic and optical properties, and prospects for future advances in electronics and optoelectronics.
- 12Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L. J.; Loh, K. P.; Zhang, H. The Chemistry of Two-dimensional Layered Transition Metal Dichalcogenide Nanosheets Nat. Chem. 2013, 5, 263– 275Google Scholar12https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BC3svotVamsA%253D%253D&md5=3dbc30db63b6a32231ecf5cae8a71cd9The chemistry of two-dimensional layered transition metal dichalcogenide nanosheetsChhowalla Manish; Shin Hyeon Suk; Eda Goki; Li Lain-Jong; Loh Kian Ping; Zhang HuaNature chemistry (2013), 5 (4), 263-75 ISSN:.Ultrathin two-dimensional nanosheets of layered transition metal dichalcogenides (TMDs) are fundamentally and technologically intriguing. In contrast to the graphene sheet, they are chemically versatile. Mono- or few-layered TMDs - obtained either through exfoliation of bulk materials or bottom-up syntheses - are direct-gap semiconductors whose bandgap energy, as well as carrier type (n- or p-type), varies between compounds depending on their composition, structure and dimensionality. In this Review, we describe how the tunable electronic structure of TMDs makes them attractive for a variety of applications. They have been investigated as chemically active electrocatalysts for hydrogen evolution and hydrosulfurization, as well as electrically active materials in opto-electronics. Their morphologies and properties are also useful for energy storage applications such as electrodes for Li-ion batteries and supercapacitors.
- 13Jariwala, D.; Sangwan, V. K.; Lauhon, L. J.; Marks, T. J.; Hersam, M. C. Emerging Device Applications for Semiconducting Two-dimensional Transition Metal Dichalcogenides ACS Nano 2014, 8, 1102– 1120Google Scholar13https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhsVKlt78%253D&md5=185f8627d2582249825fe739fcee19efEmerging Device Applications for Semiconducting Two-Dimensional Transition Metal DichalcogenidesJariwala, Deep; Sangwan, Vinod K.; Lauhon, Lincoln J.; Marks, Tobin J.; Hersam, Mark C.ACS Nano (2014), 8 (2), 1102-1120CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)A review. With advances in exfoliation and synthetic techniques, atomically thin films of semiconducting transition metal dichalcogenides have recently been isolated and characterized. Their two-dimensional structure, coupled with a direct band gap in the visible portion of the electromagnetic spectrum, suggests suitability for digital electronics and optoelectronics. Toward that end, several classes of high-performance devices are reported along with significant progress in understanding their phys. properties. Here, the authors present a review of the architecture, operating principles, and physics of electronic and optoelectronic devices based on ultrathin transition metal dichalcogenide semiconductors. By critically assessing and comparing the performance of these devices with competing technologies, the merits and shortcomings of this emerging class of electronic materials are identified, thereby providing a road map for future development.
- 14Coehoorn, R.; Haas, C.; Degroot, R. A. Electronic-structure of MoSe2, MoS2, and WSe2. II The Nature of the Optical Band-gaps Phys. Rev. B: Condens. Matter Mater. Phys. 1987, 35, 6203– 6206Google ScholarThere is no corresponding record for this reference.
- 15Kim, Y.; Huang, J. L.; Lieber, C. M. Characterization of Nanometer Scale Wear and Oxidation of Transition-metal Dichalcogenide Lubricants by Atomic Force Microscopy Appl. Phys. Lett. 1991, 59, 3404– 3406Google ScholarThere is no corresponding record for this reference.
- 16Karunadasa, H. I.; Montalvo, E.; Sun, Y. J.; Majda, M.; Long, J. R.; Chang, C. J. A Molecular MoS2 Edge Site Mimic for Catalytic Hydrogen Generation Science 2012, 335, 698– 702Google Scholar16https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhvVCjsr8%253D&md5=66849a25acaecaf60e1922de5397b061A Molecular MoS2 Edge Site Mimic for Catalytic Hydrogen GenerationKarunadasa, Hemamala I.; Montalvo, Elizabeth; Sun, Yujie; Majda, Marcin; Long, Jeffrey R.; Chang, Christopher J.Science (Washington, DC, United States) (2012), 335 (6069), 698-702CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)Inorg. solids are an important class of catalysts that often derive their activity from sparse active sites that are structurally distinct from the inactive bulk. Rationally optimizing activity is therefore beholden to the challenges in studying these active sites in mol. detail. Here, we report a mol. that mimics the structure of the proposed triangular active edge site fragments of molybdenum disulfide (MoS2), a widely used industrial catalyst that has shown promise as a low-cost alternative to platinum for electrocatalytic hydrogen prodn. By leveraging the robust coordination environment of a pentapyridyl ligand, we synthesized and structurally characterized a well-defined MoIV-disulfide complex that, upon electrochem. redn., can catalytically generate hydrogen from acidic org. media as well as from acidic water.
- 17Splendiani, A.; Sun, L.; Zhang, Y. B.; Li, T. S.; Kim, J.; Chim, C. Y.; Galli, G.; Wang, F. Emerging Photoluminescence in Monolayer MoS2 Nano Lett. 2010, 10, 1271– 1275Google Scholar17https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXjt1Sqsbs%253D&md5=7df269b35ce26d97dd8fbec1d8b6117dEmerging Photoluminescence in Monolayer MoS2Splendiani, Andrea; Sun, Liang; Zhang, Yuanbo; Li, Tianshu; Kim, Jonghwan; Chim, Chi-Yung; Galli, Giulia; Wang, FengNano Letters (2010), 10 (4), 1271-1275CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)Novel phys. phenomena can emerge in low-dimensional nanomaterials. Bulk MoS2, a prototypical metal dichalcogenide, is an indirect bandgap semiconductor with negligible photoluminescence. When the MoS2 crystal is thinned to monolayer, however, a strong photoluminescence emerges, indicating an indirect to direct bandgap transition in this d-electron system. Quantum confinement in layered d-electron materials like MoS2 provides new opportunities for engineering the electronic structure of matter at the nanoscale.
- 18Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Atomically Thin MoS2: A New Direct-gap Semiconductor Phys. Rev. Lett. 2010, 105, 136805Google Scholar18https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXht1Chs7zL&md5=f29a2e9692fc341d1b921f7862cf4c2aAtomically Thin MoS2. A New Direct-Gap SemiconductorMak, Kin Fai; Lee, Changgu; Hone, James; Shan, Jie; Heinz, Tony F.Physical Review Letters (2010), 105 (13), 136805/1-136805/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)The electronic properties of ultrathin crystals of MoS2 consisting of N = 1, 2,...,6 S-Mo-S monolayers were investigated by optical spectroscopy. Through characterization by absorption, photoluminescence, and photocond. spectroscopy, we trace the effect of quantum confinement on the material's electronic structure. With decreasing thickness, the indirect band gap, which lies below the direct gap in the bulk material, shifts upwards in energy by >0.6 eV. This leads to a crossover to a direct-gap material in the limit of the single monolayer. Unlike the bulk material, the MoS2 monolayer emits light strongly. The freestanding monolayer exhibits an increase in luminescence quantum efficiency by more than a factor of 104 compared with the bulk material.
- 19Yin, Z. Y.; Li, H.; Li, H.; Jiang, L.; Shi, Y. M.; Sun, Y. H.; Lu, G.; Zhang, Q.; Chen, X. D.; Zhang, H. Single-layer MoS2 Phototransistors ACS Nano 2012, 6, 74– 80Google Scholar19https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXhs1SmurvE&md5=28f349c4ed94a33568324f610b2dd126Single-Layer MoS2 PhototransistorsYin, Zongyou; Li, Hai; Li, Hong; Jiang, Lin; Shi, Yumeng; Sun, Yinghui; Lu, Gang; Zhang, Qing; Chen, Xiaodong; Zhang, HuaACS Nano (2012), 6 (1), 74-80CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)A new phototransistor based on the mech. exfoliated single-layer MoS2 nanosheet is fabricated, and its light-induced elec. properties were studied. Photocurrent generated from the phototransistor is solely detd. by the illuminated optical power at a const. drain or gate voltage. The switching behavior of photocurrent generation and annihilation can be completely finished within ∼50 ms, and it shows good stability. Esp., the single-layer MoS2 phototransistor exhibits a better photoresponsivity as compared with the graphene-based device. The unique characteristics of incident-light control, prompt photoswitching, and good photoresponsivity from the MoS2 phototransistor pave an avenue to develop the single-layer semiconducting materials for multifunctional optoelectronic device applications in the future.
- 20Choi, W.; Cho, M. Y.; Konar, A.; Lee, J. H.; Cha, G. B.; Hong, S. C.; Kim, S.; Kim, J.; Jena, D.; Joo, J.et al. High-detectivity Multilayer MoS2 Phototransistors with Spectral Response from Ultraviolet to Infrared Adv. Mater. 2012, 24, 5832– 5836Google Scholar20https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38Xht1aqt73E&md5=e1d2c1ec62f3f02c6c6c83fa25851ceaHigh-detectivity multilayer MoS2 phototransistors with spectral response from ultraviolet to infraredChoi, Woong; Cho, Mi Yeon; Konar, Aniruddha; Lee, Jong Hak; Cha, Gi-Beom; Hong, Soon Cheol; Kim, Sangsig; Kim, Jeongyong; Jena, Debdeep; Joo, Jinsoo; Kim, SunkookAdvanced Materials (Weinheim, Germany) (2012), 24 (43), 5832-5836CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)This study relatest to the optoelectronic properties of multilayer MoS2 TFTs and shows a compelling case of multilayer MoS2 phototransistors for applications in photodetectors. In particular, the interesting optoelectronic properties of our multilayer MoS2 phototransistors could potentially lead to their integration into touch screen panels for flat panel or flexible display devices.
- 21Lopez-Sanchez, O.; Lembke, D.; Kayci, M.; Radenovic, A.; Kis, A. Ultrasensitive Photodetectors Based on Monolayer MoS2 Nat. Nanotechnol. 2013, 8, 497– 501Google Scholar21https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXptVanurY%253D&md5=10032f352964457aa0fc6b8ee9d4a486Ultrasensitive photodetectors based on monolayer MoS2Lopez-Sanchez, Oriol; Lembke, Dominik; Kayci, Metin; Radenovic, Aleksandra; Kis, AndrasNature Nanotechnology (2013), 8 (7), 497-501CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)Ultrasensitive monolayer MoS2 phototransistors with improved device mobility and ON current are demonstrated. The devices show a max. external photoresponsivity of 880 A W-1 at λ = 561 nm and a photoresponse at 400-680 nm. With recent developments in large-scale prodn. techniques such as liq.-scale exfoliation and CVD-like growth, MoS2 shows important potential for applications in MoS2-based integrated optoelectronic circuits, light sensing, biomedical imaging, video recording and spectroscopy.
- 22Mak, K. F.; He, K. L.; Lee, C.; Lee, G. H.; Hone, J.; Heinz, T. F.; Shan, J. Tightly Bound Trions in Monolayer MoS2 Nat. Mater. 2013, 12, 207– 211Google Scholar22https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhslGku7jJ&md5=df9e334599ae4b69e243ff181e894daaTightly bound trions in monolayer MoS2Mak, Kin Fai; He, Keliang; Lee, Changgu; Lee, Gwan Hyoung; Hone, James; Heinz, Tony F.; Shan, JieNature Materials (2013), 12 (3), 207-211CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)Two-dimensional (2D) at. crystals, such as graphene and transition-metal dichalcogenides, have emerged as a new class of materials with remarkable phys. properties. In contrast to graphene, monolayer MoS2 is a noncentrosym. material with a direct energy gap. Strong photoluminescence, a current on/off ratio exceeding 108 in field-effect transistors, and efficient valley and spin control by optical helicity have recently been demonstrated in this material. Here the authors report the spectroscopic identification in a monolayer MoS2 field-effect transistor of tightly bound neg. trions, a quasiparticle composed of 2 electrons and a hole. These quasiparticles, which can be optically created with valley and spin polarized holes, have no analog in conventional semiconductors. They also possess a large binding energy (∼ 20 meV), rendering them significant even at room temp. Results open up possibilities both for fundamental studies of many-body interactions and for optoelectronic and valleytronic applications in 2-dimensional at. crystals.
- 23Coleman, J. N.; Lotya, M.; O’Neill, A.; Bergin, S. D.; King, P. J.; Khan, U.; Young, K.; Gaucher, A.; De, S.; Smith, R. J.et al. Two-dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials Science 2011, 331, 568– 571Google Scholar23https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXhtlWisLY%253D&md5=7bd4a9da1b4f81f2caa3d1159dd8a5c7Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered MaterialsColeman, Jonathan N.; Lotya, Mustafa; O'Neill, Arlene; Bergin, Shane D.; King, Paul J.; Khan, Umar; Young, Karen; Gaucher, Alexandre; De, Sukanta; Smith, Ronan J.; Shvets, Igor V.; Arora, Sunil K.; Stanton, George; Kim, Hye-Young; Lee, Kangho; Kim, Gyu Tae; Duesberg, Georg S.; Hallam, Toby; Boland, John J.; Wang, Jing Jing; Donegan, John F.; Grunlan, Jaime C.; Moriarty, Gregory; Shmeliov, Aleksey; Nicholls, Rebecca J.; Perkins, James M.; Grieveson, Eleanor M.; Theuwissen, Koenraad; McComb, David W.; Nellist, Peter D.; Nicolosi, ValeriaScience (Washington, DC, United States) (2011), 331 (6017), 568-571CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)If they could be easily exfoliated, layered materials would become a diverse source of two-dimensional crystals whose properties would be useful in applications ranging from electronics to energy storage. Layered compds. such as MoS2, WS2, MoSe2, MoTe2, TaSe2, NbSe2, NiTe2, BN, and Bi2Te3 can be efficiently dispersed in common solvents and can be deposited as individual flakes or formed into films. Electron microscopy strongly suggests that the material is exfoliated into individual layers. By blending this material with suspensions of other nanomaterials or polymer solns., the authors can prep. hybrid dispersions or composites, which can be cast into films. WS2 and MoS2 effectively reinforce polymers, whereas WS2/carbon nanotube hybrid films have high cond., leading to promising thermoelec. properties.
- 24Wang, H.; Yu, L. L.; Lee, Y. H.; Shi, Y. M.; Hsu, A.; Chin, M. L.; Li, L. J.; Dubey, M.; Kong, J.; Palacios, T. Integrated Circuits Based on Bilayer MoS2 Transistors Nano Lett. 2012, 12, 4674– 4680Google Scholar24https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhtFCht7jL&md5=36edd940f16c94fe388951be66b46e40Integrated Circuits Based on Bilayer MoS2 TransistorsWang, Han; Yu, Lili; Lee, Yi-Hsien; Shi, Yumeng; Hsu, Allen; Chin, Matthew L.; Li, Lain-Jong; Dubey, Madan; Kong, Jing; Palacios, TomasNano Letters (2012), 12 (9), 4674-4680CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)Two-dimensional (2D) materials, such as molybdenum disulfide (MoS2), have been shown to exhibit excellent elec. and optical properties. The semiconducting nature of MoS2 allows it to overcome the shortcomings of zero-bandgap graphene, while still sharing many of graphene's advantages for electronic and optoelectronic applications. Discrete electronic and optoelectronic components, such as field-effect transistors, sensors, and photodetectors made from few-layer MoS2 show promising performance as potential substitute of Si in conventional electronics and of org. and amorphous Si semiconductors in ubiquitous systems and display applications. An important next step is the fabrication of fully integrated multistage circuits and logic building blocks on MoS2 to demonstrate its capability for complex digital logic and high-frequency ac applications. This paper demonstrates an inverter, a NAND gate, a static random access memory, and a five-stage ring oscillator based on a direct-coupled transistor logic technol. The circuits comprise between 2 to 12 transistors seamlessly integrated side-by-side on a single sheet of bilayer MoS2. Both enhancement-mode and depletion-mode transistors were fabricated thanks to the use of gate metals with different work functions.
- 25Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-layer MoS2 Transistors Nat. Nanotechnol. 2011, 6, 147– 150Google Scholar25https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXislCjsro%253D&md5=555366539a8a87d074a69674aafaf315Single-layer MoS2 transistorsRadisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A.Nature Nanotechnology (2011), 6 (3), 147-150CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)Two-dimensional materials are attractive for use in next-generation nanoelectronic devices because, compared to 1D materials, it is relatively easy to fabricate complex structures from them. The most widely studied 2D material is graphene, both because of its rich physics and its high mobility. However, pristine graphene does not have a bandgap, a property that is essential for many applications, including transistors. Engineering a graphene bandgap increases fabrication complexity and either reduces mobilities to the level of strained Si films or requires high voltages. Although single layers of MoS2 have a large intrinsic bandgap of 1.8 eV, previously reported mobilities in the 0.5-3 cm2 V-1 s-1 range are too low for practical devices. Here, we use a HfO2 gate dielec. to demonstrate a room-temp. single-layer MoS2 mobility of at least 200 cm2 V-1 s-1, similar to that of graphene nanoribbons, and demonstrate transistors with room-temp. current on/off ratios of 1 × 108 and ultralow standby power dissipation. Because monolayer MoS2 has a direct bandgap, it can be used to construct interband tunnel FETs, which offer lower power consumption than classical transistors. Monolayer MoS2 could also complement graphene in applications that require thin transparent semiconductors, such as optoelectronics and energy harvesting.
- 26Alam, K.; Lake, R. K. Monolayer MoS2 Transistors beyond the Technology Road Map IEEE Trans. Electron Devices 2012, 59, 3250– 3254Google ScholarThere is no corresponding record for this reference.
- 27Radisavljevic, B.; Whitwick, M. B.; Kis, A. Small-signal Amplifier Based On Single-layer MoS2 Appl. Phys. Lett. 2012, 101, 043103Google Scholar27https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhtVOgur7L&md5=a92ebe5c74d0b5e26a12d76e9324576dSmall-signal amplifier based on single-layer MoS2Radisavljevic, Branimir; Whitwick, Michael B.; Kis, AndrasApplied Physics Letters (2012), 101 (4), 043103/1-043103/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)We demonstrate the operation of an analog small-signal amplifier based on single-layer MoS2, a semiconducting analog of graphene. Our device consists of 2 transistors integrated on the same piece of single-layer MoS2. The high intrinsic band gap of 1.8 eV allows MoS2-based amplifiers to operate with a room temp. gain of 4. The amplifier operation is demonstrated for the frequencies of input signal up to 2 kHz preserving the gain higher than 1. MoS2 can effectively amplify signals and that it could be used for advanced analog circuits based on 2D materials. (c) 2012 American Institute of Physics.
- 28Radisavljevic, B.; Kis, A. Mobility Engineering and a Metal-insulator Transition in Monolayer MoS2 Nat. Mater. 2013, 12, 815– 820Google Scholar28https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXpvVyqsbg%253D&md5=4e7fd06bc5f8097ac80f5978694581abMobility engineering and a metal-insulator transition in monolayer MoS2Radisavljevic, Branimir; Kis, AndrasNature Materials (2013), 12 (9), 815-820CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)Two-dimensional (2D) materials are a new class of materials with interesting phys. properties and applications ranging from nanoelectronics to sensing and photonics. In addn. to graphene, the most studied 2-dimensional material, monolayers of other layered materials such as semiconducting dichalcogenides MoS2 or WSe2 are gaining in importance as promising channel materials for field-effect transistors (FETs). The presence of a direct bandgap in monolayer MoS2 due to quantum-mech. confinement allows room-temp. FETs with an on/off ratio exceeding 108. The presence of high-κ dielecs. in these devices enhanced their mobility, but the mechanisms are not well understood. Here, the authors report on elec. transport measurements on MoS2 FETs in different dielec. configurations. The dependence of mobility on temp. shows clear evidence of the strong suppression of charged-impurity scattering in dual-gate devices with a top-gate dielec. At the same time, phonon scattering shows a weaker than expected temp. dependence. High levels of doping achieved in dual-gate devices also allow the observation of a metal-insulator transition in monolayer MoS2 due to strong electron-electron interactions. The authors' work opens up the way to further improvements in 2-dimensional semiconductor performance and introduces MoS2 as an interesting system for studying correlation effects in mesoscopic systems.
- 29Zhang, W. J.; Chuu, C. P.; Huang, J. K.; Chen, C. H.; Tsai, M. L.; Chang, Y. H.; Liang, C. T.; Chen, Y. Z.; Chueh, Y. L.; He, J. H.et al. Ultrahigh-gain Photodetectors Based on Atomically Thin Graphene-MoS2 Heterostructures Sci. Rep. 2014, 4, 3826Google Scholar29https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXjslCis7c%253D&md5=721ff0731857e8c68d85a5ae137fcb31Ultrahigh-Gain Photodetectors Based on Atomically Thin Graphene-MoS2 HeterostructuresZhang, Wenjing; Chuu, Chih-Piao; Huang, Jing-Kai; Chen, Chang-Hsiao; Tsai, Meng-Lin; Chang, Yung-Huang; Liang, Chi-Te; Chen, Yu-Ze; Chueh, Yu-Lun; He, Jr-Hau; Chou, Mei-Yin; Li, Lain-JongScientific Reports (2014), 4 (), 3826/1-3826/8CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)Due to its high carrier mobility, broadband absorption, and fast response time, the semi-metallic graphene is attractive for optoelectronics. Another two-dimensional semiconducting material molybdenum disulfide (MoS2) is also known as light- sensitive. Here we show that a large-area and continuous MoS2 monolayer is achievable using a CVD method and graphene is transferable onto MoS2. We demonstrate that a photodetector based on the graphene/MoS2 heterostructure is able to provide a high photogain greater than 108. Our expts. show that the electron-hole pairs are produced in the MoS2 layer after light absorption and subsequently sepd. across the layers. Contradictory to the expectation based on the conventional built-in elec. field model for metal-semiconductor contacts, photoelectrons are injected into the graphene layer rather than trapped in MoS2 due to the presence of a perpendicular effective elec. field caused by the combination of the built-in elec. field, the applied electrostatic field, and charged impurities or adsorbates, resulting in a tuneable photoresponsivity.
- 30Bertolazzi, S.; Krasnozhon, D.; Kis, A.; Nonvolatile Memory Cells Based On MoS2/Graphene Heterostructures ACS Nano 2013, 7, 3246– 3252Google ScholarThere is no corresponding record for this reference.
- 31Kaasbjerg, K.; Thygesen, K. S.; Jacobsen, K. W. Phonon-limited Mobility In N-type Single-layer MoS2 from First Principles Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 115317Google Scholar31https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38Xpt1ynu7c%253D&md5=61d96a0b9479a4c9c5241b42609fcdd3Phonon-limited mobility in n-type single-layer MoS2 from first principlesKaasbjerg, Kristen; Thygesen, Kristian S.; Jacobsen, Karsten W.Physical Review B: Condensed Matter and Materials Physics (2012), 85 (11), 115317/1-115317/16CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)We study the phonon-limited mobility in intrinsic n-type single-layer MoS2 for temps. T > 100 K. The materials properties including the electron-phonon interaction are calcd. from first principles and the deformation potentials and Frohlich interaction in single-layer MoS2 are established. The calcd. room-temp. mobility of ∼410 cm2V-1s-1 is found to be dominated by optical phonon scattering via intra and intervalley deformation potential couplings and the Frohlich interaction. The mobility is weakly dependent on the carrier d. and follows a μ ∼ T-γ temp. dependence with γ = 1.69 at room temp. It is shown that a quenching of the characteristic homopolar mode, which is likely to occur in top-gated samples, increases the mobility with ∼70 cm2V-1s-1 and can be obsd. as a decrease in the exponent to γ = 1.52. In comparison to recent exptl. findings for the mobility in single-layer MoS2 (∼200 cm2V-1s-1), our results indicate that mobilities close to the intrinsic phonon-limited mobility can be achieved in two-dimensional materials via dielec. engineering that effectively screens static Coulomb scattering on, e.g., charged impurities.
- 32Das, S.; Chen, H. Y.; Penumatcha, A. V.; Appenzeller, J. High Performance Multilayer MoS2 Transistors with Scandium Contacts Nano Lett. 2013, 13, 100– 105Google Scholar32https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhvVersLzO&md5=e5e71a0be2d5d45f7cc4ff9149b3cbb4High Performance Multilayer MoS2 Transistors with Scandium ContactsDas, Saptarshi; Chen, Hong-Yan; Penumatcha, Ashish Verma; Appenzeller, JoergNano Letters (2013), 13 (1), 100-105CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)While there was growing interest in two-dimensional (2-D) crystals other than graphene, evaluating their potential usefulness for electronic applications is still in its infancy due to the lack of a complete picture of their performance potential. The focus of this article is on contacts. Through a proper understanding and design of source/drain contacts and the right choice of no. of MoS2 layers the excellent intrinsic properties of this 2-dimensional material can be harvested. Using scandium contacts on 10-nm-thick exfoliated MoS2 flakes that are covered by a 15. nm Al2O3 film, high effective mobilities of 700 cm2/(V s) are achieved at room temp. This breakthrough is largely attributed to the fact that the authors succeeded in eliminating contact resistance effects that limited the device performance in the past unrecognized. In fact, the apparent linear dependence of current on drain voltage had mislead researchers to believe that a truly ohmic contact had already been achieved, a misconception that the authors also elucidate.
- 33Shi, H.; Pan, H.; Zhang, Y. W.; Yakobson, B. I. Quasiparticle Band Structures and Optical Properties of Strained Monolayer MoS2 and WS2 Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87, 155304Google Scholar33https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXovVens78%253D&md5=2bdbe3b5eb1a87afbe3d90cb73ca70ecQuasiparticle band structures and optical properties of strained monolayer MoS2 and WS2Shi, Hongliang; Pan, Hui; Zhang, Yong-Wei; Yakobson, Boris I.Physical Review B: Condensed Matter and Materials Physics (2013), 87 (15), 155304/1-155304/8CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)The quasiparticle (QP) band structures of both strainless and strained monolayer MoS2 are investigated using more accurate many-body perturbation GW theory and maximally localized Wannier functions (MLWFs) approach. By solving the Bethe-Salpeter equation (BSE) including excitonic effects on top of the partially self-consistent GW0 (scGW0) calcn., the predicted optical gap magnitude is in good agreement with available exptl. data. With increasing strain, the exciton binding energy is nearly unchanged, while optical gap is reduced significantly. The scGW0 and BSE calcns. are also performed on monolayer WS2, similar characteristics are predicted and WS2 possesses the lightest effective mass at the same strain among monolayers Mo(S,Se) and W(S,Se). Our results also show that the electron effective mass decreases as the tensile strain increases, resulting in an enhanced carrier mobility. The present calcn. results suggest a viable route to tune the electronic properties of monolayer transition-metal dichalcogenides (TMDs) using strain engineering for potential applications in high performance electronic devices.
- 34Allain, A.; Kis, A. Electron and Hole Mobilities in Single-layer WSe2 ACS Nano 2014, 8, 7180– 7185Google Scholar34https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhtVahtrrF&md5=ff021e02a26db6df8981fc27101b2e3cElectron and Hole Mobilities in Single-Layer WSe2Allain, Adrien; Kis, AndrasACS Nano (2014), 8 (7), 7180-7185CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)Single-layer transition metal dichalcogenide WSe2 has recently attracted a lot of attention because it is a 2-dimensional semiconductor with a direct band gap. Due to low doping levels, it is intrinsic and shows ambipolar transport. This opens up the possibility to realize devices with the Fermi level located in the valence band, where the spin/valley coupling is strong and leads to new and interesting physics. As a consequence of its intrinsically low doping, large Schottky barriers form between WSe2 and metal contacts, which impede the injection of charges at low temps. Here, the authors report on the study of single-layer WSe2 transistors with a polymer electrolyte gate (PEO: LiClO4). Polymer electrolytes allow the charge carrier densities to be modulated to very high values, allowing the observation of both the electron- and the hole-doped regimes. Also, the authors' ohmic contacts formed at low temps. allow the authors to study the temp. dependence of electron and hole mobilities. At high electron densities, a reentrant insulating regime is also obsd., a feature which is absent at high hole densities.
- 35Huang, J. K.; Pu, J.; Hsu, C. L.; Chiu, M. H.; Juang, Z. Y.; Chang, Y. H.; Chang, W. H.; Iwasa, Y.; Takenobu, T.; Li, L. J. Large-area Synthesis of Highly Crystalline WSe2 Mono Layers and Device Applications ACS Nano 2014, 8, 923– 930Google ScholarThere is no corresponding record for this reference.
- 36Jones, A. M.; Yu, H. Y.; Ghimire, N. J.; Wu, S. F.; Aivazian, G.; Ross, J. S.; Zhao, B.; Yan, J. Q.; Mandrus, D. G.; Xiao, D.et al. Optical Generation of Excitonic Valley Coherence in Monolayer WSe2 Nat. Nanotechnol. 2013, 8, 634Google Scholar36https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXht1CgsLjE&md5=d6f50e4ea06c115268e08c4e324b35d7Optical generation of excitonic valley coherence in monolayer WSe2Jones, Aaron M.; Yu, Hongyi; Ghimire, Nirmal J.; Wu, Sanfeng; Aivazian, Grant; Ross, Jason S.; Zhao, Bo; Yan, Jiaqiang; Mandrus, David G.; Xiao, Di; Yao, Wang; Xu, XiaodongNature Nanotechnology (2013), 8 (9), 634-638CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)As a consequence of degeneracies arising from crystal symmetries, it is possible for electron states at band-edges ( valleys') to have addnl. spin-like quantum nos. An important question is whether coherent manipulation can be performed on such valley pseudospins, analogous to that implemented using true spin, in the quest for quantum technologies. Here, we show that valley coherence can be generated and detected. Because excitons in a single valley emit circularly polarized photons, linear polarization can only be generated through recombination of an exciton in a coherent superposition of the two valley states. Using monolayer semiconductor WSe2 devices, we first establish the circularly polarized optical selection rules for addressing individual valley excitons and trions. We then demonstrate coherence between valley excitons through the observation of linearly polarized luminescence, whose orientation coincides with that of the linearly polarized excitation, for any given polarization angle. In contrast, the corresponding photoluminescence from trions is not obsd. to be linearly polarized, consistent with the expectation that the emitted photon polarization is entangled with valley pseudospin. The ability to address coherence, in addn. to valley polarization, is a step forward towards achieving quantum manipulation of the valley index necessary for coherent valleytronics.
- 37Yoshida, S.; Terada, Y.; Yokota, M.; Takeuchi, O.; Meray, Y.; Shigekawa, H. Direct Probing of Transient Photocurrent Dynamics in P-WSe2 By Time-resolved Scanning Tunneling Microscopy Appl. Phys. Express 2013, 6, 016601Google ScholarThere is no corresponding record for this reference.
- 38Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect In Atomically Thin Carbon Films Science 2004, 306, 666– 669Google Scholar38https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2cXos1Kqt70%253D&md5=488da13500bf24e8fc419052dc1a9e84Electric Field Effect in Atomically Thin Carbon FilmsNovoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A.Science (Washington, DC, United States) (2004), 306 (5696), 666-669CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)The authors describe monocryst. graphitic films, which are a few atoms thick but are nonetheless stable under ambient conditions, metallic, and of remarkably high quality. The films are a two-dimensional semimetal with a tiny overlap between valence and conductance bands, and they exhibit a strong ambipolar elec. field effect such that electrons and holes in concns. up to 1013 per square centimeter and with room-temp. mobilities of ∼10,000 square centimeters per V-second can be induced by applying gate voltage.
- 39Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. Two-dimensional Gas of Massless Dirac Fermions in Graphene Nature 2005, 438, 197– 200Google Scholar39https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2MXhtF2nsrnI&md5=56138229370ff26ece1857a049f00f53Two-dimensional gas of massless Dirac fermions in grapheneNovoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A.Nature (London, United Kingdom) (2005), 438 (7065), 197-200CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)Quantum electrodynamics (resulting from the merger of quantum mechanics and relativity theory) has provided a clear understanding of phenomena ranging from particle physics to cosmol. and from astrophysics to quantum chem. The ideas underlying quantum electrodynamics also influence the theory of condensed matter, but quantum relativistic effects are usually minute in the known exptl. systems that can be described accurately by the non-relativistic Schroedinger equation. Here we report an exptl. study of a condensed-matter system (graphene, a single at. layer of carbon) in which electron transport is essentially governed by Dirac's (relativistic) equation. The charge carriers in graphene mimic relativistic particles with zero rest mass and have an effective 'speed of light' c* ≈ 106 m s-1. Our study reveals a variety of unusual phenomena that are characteristic of two-dimensional Dirac fermions. In particular we have obsd. the following: first, graphene's cond. never falls below a min. value corresponding to the quantum unit of conductance, even when concns. of charge carriers tend to zero; second, the integer quantum Hall effect in graphene is anomalous in that it occurs at half-integer filling factors; and third, the cyclotron mass mc of massless carriers in graphene is described by E = mcc*2. This two-dimensional system is not only interesting in itself but also allows access to the subtle and rich physics of quantum electrodynamics in a bench-top expt.
- 40Tonndorf, P.; Schmidt, R.; Bottger, P.; Zhang, X.; Borner, J.; Liebig, A.; Albrecht, M.; Kloc, C.; Gordan, O.; Zahn, D. R. T.et al. Photoluminescence Emission and Raman Response of Monolayer MoS2, MoSe2, And WSe2 Opt. Express 2013, 21, 4908– 4916Google Scholar40https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXjslyrurY%253D&md5=d17f7bdbba58f863220eefdeb116e5caPhotoluminescence emission and Raman response of monolayer MoS2, MoSe2, and WSe2Tonndorf, Philipp; Schmidt, Robert; Boettger, Philipp; Zhang, Xiao; Boerner, Janna; Liebig, Andreas; Albrecht, Manfred; Kloc, Christian; Gordan, Ovidiu; Zahn, Dietrich R. T.; de Vasconcellos, Steffen Michaelis; Bratschitsch, RudolfOptics Express (2013), 21 (4), 4908-4916CODEN: OPEXFF; ISSN:1094-4087. (Optical Society of America)We mech. exfoliate mono- and few-layers of the transition metal dichalcogenides molybdenum disulfide, molybdenum diselenide, and tungsten diselenide. The exact no. of layers is unambiguously detd. by at. force microscopy and high-resoln. Raman spectroscopy. Strong photoluminescence emission is caused by the transition from an indirect band gap semiconductor of bulk material to a direct band gap semiconductor in atomically thin form.
- 41Zhao, W. J.; Ghorannevis, Z.; Chu, L. Q.; Toh, M. L.; Kloc, C.; Tan, P. H.; Eda, G. Evolution of Electronic Structure in Atomically Thin Sheets of WS2 and WSe2 ACS Nano 2013, 7, 791– 797Google Scholar41https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhvVGrur%252FM&md5=1077846a4fad1eb6029ced7f87b1a708Evolution of Electronic Structure in Atomically Thin Sheets of WS2 and WSe2Zhao, Weijie; Ghorannevis, Zohreh; Chu, Leiqiang; Toh, Minglin; Kloc, Christian; Tan, Ping-Heng; Eda, GokiACS Nano (2013), 7 (1), 791-797CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)Geometrical confinement effect in exfoliated sheets of layered materials leads to significant evolution of energy dispersion in mono- to few-layer thickness regime. Molybdenum disulfide (MoS2) was recently found to exhibit indirect-to-direct gap transition when the thickness is reduced to a single monolayer. Emerging photoluminescence (PL) from monolayer MoS2 opens up opportunities for a range of novel optoelectronic applications of the material. Here we report differential reflectance and PL spectra of mono- to few-layer WS2 and WSe2 that indicate that the band structure of these materials undergoes similar indirect-to-direct gap transition when thinned to a single monolayer. The transition is evidenced by distinctly enhanced PL peak centered at 630 and 750 nm in monolayer WS2 and WSe2, resp. Few-layer flakes are found to exhibit comparatively strong indirect gap emission along with direct gap hot electron emission, suggesting high quality of synthetic crystals prepd. by a chem. vapor transport method. Fine absorption and emission features and their thickness dependence suggest a strong effect of Se p-orbitals on the d electron band structure as well as interlayer coupling in WSe2.
- 42Lee, Y. H.; Zhang, X. Q.; Zhang, W. J.; Chang, M. T.; Lin, C. T.; Chang, K. D.; Yu, Y. C.; Wang, J. T. W.; Chang, C. S.; Li, L. J.et al. Synthesis of Large-area MoS2 Atomic Layers with Chemical Vapor Deposition Adv. Mater. 2012, 24, 2320– 2325Google Scholar42https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XkvVKiur8%253D&md5=7b5ca7016ced6baa546a93be5bbe8589Synthesis of Large-Area MoS2 Atomic Layers with Chemical Vapor DepositionLee, Yi-Hsien; Zhang, Xin-Quan; Zhang, Wenjing; Chang, Mu-Tung; Lin, Cheng-Te; Chang, Kai-Di; Yu, Ya-Chu; Wang, Jacob Tse-Wei; Chang, Chia-Seng; Li, Lain-Jong; Lin, Tsung-WuAdvanced Materials (Weinheim, Germany) (2012), 24 (17), 2320-2325CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)Large-area MoS2 at. layers are synthesized on SiO2 substrates by chem. vapor deposition using MoO3 and S powders as the reactants. Optical, microscopic and elec. measurements suggest that the synthetic process leads to the growth of MoS2 monolayer. The TEM images verify that the synthesized MoS2 sheets are highly cryst. To check for elec. performance bottom-gated transistors on silica/silicon using photolithog. was fabricated directly on top of the MoS2 sheets. The transfer curve (drain current vs. gate voltage) was computed and field effect mobility was detd. from anal. of the curve.
- 43Lee, Y. H.; Yu, L.; Wang, H.; Fang, W.; I Ling, X.; Shi, Y.; Lin, C. T.; Huang, J. K.; Chang, M. T.; Chang, C. S.et al. Synthesis and Transfer of Single-layer Transition Metal Disulfides on Diverse Surfaces Nano Lett. 2013, 13, 1852Google Scholar43https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXktF2gtr8%253D&md5=c3ef496ee3dd9ec34ec96b8f1be19af2Synthesis and Transfer of Single-Layer Transition Metal Disulfides on Diverse SurfacesLee, Yi-Hsien; Yu, Lili; Wang, Han; Fang, Wenjing; Ling, Xi; Shi, Yumeng; Lin, Cheng-Te; Huang, Jing-Kai; Chang, Mu-Tung; Chang, Chia-Seng; Dresselhaus, Mildred; Palacios, Tomas; Li, Lain-Jong; Kong, JingNano Letters (2013), 13 (4), 1852-1857CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)High-quality MS2 (M = Mo, W) monolayers have been prepd. using ambient-pressure chem. vapor deposition (APCVD) with the seeding of perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS). The growth of a MS2 monolayer is achieved on various surfaces with a significant flexibility to surface corrugation. Electronic transport and optical performances of the as-grown MS2 monolayers are comparable to those of exfoliated MS2 monolayers. A robust technique for transferring the MS2 monolayer samples to diverse surfaces was developed which may stimulate the progress on the class of materials and open a new route toward the synthesis of various novel hybrid structures with LTMD monolayer and functional materials.
- 44Liu, K. K.; Zhang, W. J.; Lee, Y. H.; Lin, Y. C.; Chang, M. T.; Su, C.; Chang, C. S.; Li, H.; Shi, Y. M.; Zhang, H.et al. Growth of Large-area And Highly Crystalline MoS2 Thin Layers on Insulating Substrates Nano Lett. 2012, 12, 1538– 1544Google Scholar44https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XivVOgsbg%253D&md5=d9954476e4e5a84d99e2dd4a4290b3edGrowth of Large-Area and Highly Crystalline MoS2 Thin Layers on Insulating SubstratesLiu, Keng-Ku; Zhang, Wenjing; Lee, Yi-Hsien; Lin, Yu-Chuan; Chang, Mu-Tung; Su, Ching-Yuan; Chang, Chia-Seng; Li, Hai; Shi, Yumeng; Zhang, Hua; Lai, Chao-Sung; Li, Lain-JongNano Letters (2012), 12 (3), 1538-1544CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)The two-dimensional layer of molybdenum disulfide (MoS2) has recently attracted much interest due to its direct-gap property and potential applications in optoelectronics and energy harvesting. However, the synthetic approach to obtain high-quality and large-area MoS2 at. thin layers is still rare. Here we report that the high-temp. annealing of a thermally decompd. ammonium thiomolybdate layer in the presence of sulfur can produce large-area MoS2 thin layers with superior elec. performance on insulating substrates. Spectroscopic and microscopic results reveal that the synthesized MoS2 sheets are highly cryst. The electron mobility of the bottom-gate transistor devices made of the synthesized MoS2 layer is comparable with those of the micromechanically exfoliated thin sheets from MoS2 crystals. This synthetic approach is simple, scalable, and applicable to other transition metal dichalcogenides. Meanwhile, the obtained MoS2 films are transferable to arbitrary substrates, providing great opportunities to make layered composites by stacking various atomically thin layers.
- 45Li, X. S.; Cai, W. W.; An, J. H.; Kim, S.; Nah, J.; Yang, D. X.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.et al. Large-area Synthesis of High-quality and Uniform Graphene Films on Copper Foils Science 2009, 324, 1312– 1314Google Scholar45https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXms12gtbY%253D&md5=d5d5a8564d2dac69173cf0696d21eb3eLarge-Area Synthesis of High-Quality and Uniform Graphene Films on Copper FoilsLi, Xuesong; Cai, Weiwei; An, Jinho; Kim, Seyoung; Nah, Junghyo; Yang, Dongxing; Piner, Richard; Velamakanni, Aruna; Jung, Inhwa; Tutuc, Emanuel; Banerjee, Sanjay K.; Colombo, Luigi; Ruoff, Rodney S.Science (Washington, DC, United States) (2009), 324 (5932), 1312-1314CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)Graphene was attracting great interest because of its distinctive band structure and phys. properties. Today, graphene is limited to small sizes because it is produced mostly by exfoliating graphite. The authors grew large-area graphene films of the order of centimeters on Cu substrates by CVD using methane. The films are predominantly single-layer graphene, with a small percentage (<5%) of the area having few layers, and are continuous across Cu surface steps and grain boundaries. The low soly. of C in Cu appears to help make this growth process self-limiting. The authors also developed graphene film transfer processes to arbitrary substrates, and dual-gated field-effect transistors fabricated on Si/SiO2 substrates showed electron mobilities ≤4050 cm2/V-s at room temp.
- 46Korn, T.; Heydrich, S.; Hirmer, M.; Schmutzler, J.; Schuller, C. Low-temperature Photocarrier Dynamics in Monolayer MoS2 Appl. Phys. Lett. 2011, 99, 102109Google Scholar46https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXhtFCqtbvF&md5=dd2b95fed91ba88efdee15c6c5fe227aLow-temperature photocarrier dynamics in monolayer MoS2Korn, T.; Heydrich, S.; Hirmer, M.; Schmutzler, J.; Schueller, C.Applied Physics Letters (2011), 99 (10), 102109/1-102109/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)The band structure of MoS2 strongly depends on the no. of layers, and a transition from indirect to direct-gap semiconductor was obsd. recently for a single layer of MoS2. Single-layer MoS2 therefore becomes an efficient emitter of photoluminescence even at room temp. Here, we report on scanning Raman and on temp.-dependent, as well as time-resolved photoluminescence measurements on single-layer MoS2 flakes prepd. by exfoliation. We observe the emergence of 2 distinct photoluminescence peaks at low temps. The photocarrier recombination at low temps. occurs on the few-picosecond timescale, but with increasing temps., a biexponential photoluminescence decay with a longer-lived component is obsd. (c) 2011 American Institute of Physics.
- 47Sundaram, R.; Engel, M.; Lombardo, A.; Krupke, R.; Ferrari, A.; H Avouris, P.; Steiner, M. Electroluminescence in Single Layer MoS2 Nano Lett. 2013, 13, 1416– 1421Google Scholar47https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXktlyksbg%253D&md5=bfe4b93685b4821572c00b3e67245fb9Electroluminescence in Single Layer MoS2Sundaram, R. S.; Engel, M.; Lombardo, A.; Krupke, R.; Ferrari, A. C.; Avouris, Ph.; Steiner, M.Nano Letters (2013), 13 (4), 1416-1421CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)The authors detect electroluminescence in single layer MoS2 FETs built on transparent glass substrates. By comparing the absorption, photoluminescence, and electroluminescence of the same MoS2 layer, they all involve the same excited state at 1.8 eV. The electroluminescence has pronounced threshold behavior and is localized at the contacts. Single layer MoS2, a direct band gap semiconductor, could be promising for novel optoelectronic devices, such as 2-dimensional light detectors and emitters.
- 48Wang, R.; Ruzicka, B. A.; Kumar, N.; Bellus, M. Z.; Chiu, H. Y.; Zhao, H. Ultrafast and Spatially Resolved Studies of Charge Carriers in Atomically Thin Molybdenum Disulfide Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86, 045406Google Scholar48https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhtlSltL3O&md5=da9321359a0928768a41a180d8a5828dUltrafast and spatially resolved studies of charge carriers in atomically thin molybdenum disulfideWang, Rui; Ruzicka, Brian A.; Kumar, Nardeep; Bellus, Matthew Z.; Chiu, Hsin-Ying; Zhao, HuiPhysical Review B: Condensed Matter and Materials Physics (2012), 86 (4), 045406/1-045406/5CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)Atomically thin molybdenum disulfide is emerging as a new nanomaterial with potential applications in the fields of electronic and photonics. Charge carrier dynamics plays an essential role in detg. its electronic and optical properties. We report spatially and temporally resolved pump-probe studies of charge carriers in atomically thin molybdenum disulfide samples fabricated by mech. exfoliation. Carriers are injected by interband absorption of a 390-nm pump pulse and detected by measuring differential reflection of a time-delayed and spatially scanned probe pulse that is tuned to an exciton transition. Several parameters on charge carrier dynamics are deduced, including carrier lifetime, diffusion coeff., diffusion length, and mobility.
- 49Shi, H. Y.; Yan, R. S.; Bertolazzi, S.; Brivio, J.; Gao, B.; Kis, A.; Jena, D.; Xing, H. G.; Huang, L. B. Exciton Dynamics in Suspended Mono Layer And Few-layer MoS2 2D Crystals ACS Nano 2013, 7, 1072– 1080Google ScholarThere is no corresponding record for this reference.
- 50Lee, C.; Yan, H.; Brus, L. E.; Heinz, T. F.; Hone, J.; Ryu, S. Anomalous Lattice Vibrations of Single- and Few-layer MoS2 ACS Nano 2010, 4, 2695– 2700Google Scholar50https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXkslKktLc%253D&md5=2bf8b27f754f40e5cdf890343171555fAnomalous Lattice Vibrations of Single- and Few-Layer MoS2Lee, Changgu; Yan, Hugen; Brus, Louis E.; Heinz, Tony F.; Hone, James; Ryu, SunminACS Nano (2010), 4 (5), 2695-2700CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)Molybdenum disulfide (MoS2) of single- and few-layer thickness was exfoliated on SiO2/Si substrate and characterized by Raman spectroscopy. The no. of S-Mo-S layers of the samples was independently detd. by contact-mode at. force microscopy. Two Raman modes, E12g and A1g, exhibited sensitive thickness dependence, with the frequency of the former decreasing and that of the latter increasing with thickness. The results provide a convenient and reliable means for detg. layer thickness with at.-level precision. The opposite direction of the frequency shifts, which cannot be explained solely by van der Waals interlayer coupling, is attributed to Coulombic interactions and possible stacking-induced changes of the intralayer bonding. This work exemplifies the evolution of structural parameters in layered materials in changing from the three-dimensional to the two-dimensional regime.
- 51Ulbricht, R.; Hendry, E.; Shan, J.; Heinz, T. F.; Bonn, M. Carrier Dynamics In Semiconductors Studied with Time-resolved Terahertz Spectroscopy Rev. Mod. Phys. 2011, 83, 543– 586Google Scholar51https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXos1eitrc%253D&md5=834abf847f8a88a90b38ebd7f24e872dCarrier dynamics in semiconductors studied with time-resolved terahertz spectroscopyUlbricht, Ronald; Hendry, Euan; Shan, Jie; Heinz, Tony F.; Bonn, MischaReviews of Modern Physics (2011), 83 (2), 543-586CODEN: RMPHAT; ISSN:0034-6861. (American Physical Society)A review. Time-resolved, pulsed terahertz spectroscopy has developed into a powerful tool to study charge carrier dynamics in semiconductors and semiconductor structures over the past decades. Covering the energy range from a few to about 100 meV, terahertz radiation is sensitive to the response of charge quasiparticles, e.g., free carriers, polarons, and excitons. The distinct spectral signatures of these different quasiparticles in the THz range allow their discrimination and characterization using pulsed THz radiation. This frequency region is also well suited for the study of phonon resonances and intraband transitions in low-dimensional systems. Moreover, using a pump-probe scheme, it is possible to monitor the nonequil. time evolution of carriers and low-energy excitations with sub-ps time resoln. Being an all-optical technique, terahertz time-domain spectroscopy is contact-free and noninvasive and hence suited to probe the cond. of, particularly, nanostructured materials that are difficult or impossible to access with other methods. The latest developments in the application of terahertz time-domain spectroscopy to bulk and nanostructured semiconductors are reviewed.
- 52Lloyd-Hughes, J.; Jeon, T.-I. A Review of the Terahertz Conductivity of Bulk and Nano-materials J. Infrared, Millimeter, Terahertz Waves 2012, 33, 871Google Scholar52https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhtFSlt7rK&md5=2275551b24f861b4f325229e2989d751A Review of the Terahertz Conductivity of Bulk and Nano-MaterialsLloyd-Hughes, James; Jeon, Tae-InJournal of Infrared, Millimeter, and Terahertz Waves (2012), 33 (9), 871-925CODEN: JIMTC4; ISSN:1866-6892. (Springer)We review pioneering and recent studies of the cond. of solid state systems at terahertz frequencies. A variety of theor. formalisms that describe the terahertz cond. of bulk, mesoscopic and nanoscale materials are outlined, and their validity and limitations are given. Exptl. highlights are discussed from studies of inorg. semiconductors, org. materials (such as graphene, carbon nanotubes and polymers), metallic films and strongly correlated electron systems including superconductors.
- 53Grabtchak, S. Y.; Cocivera, M. Contactless Microwave Study of Dispersive Transport in Thin Film CdSe J. Appl. Phys. 1996, 79, 786– 793Google Scholar53https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK28XltFKhsQ%253D%253D&md5=36d886a799e2adb70ea1eab52338c0dfContactless microwave study of dispersive transport in thin film CdSeGrabtchak, Serguei Yu; Cocivera, MichaelJournal of Applied Physics (1996), 79 (2), 786-93CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)The contactless microwave technique was used to measure light-induced transients in the power absorbed by thin films of polycryst. CdSe. Because the rise time of the microwave cavity was 60 ns, the anal. was limited to 100 ns or longer. Measurement of these transients at a no. of fixed frequencies across the dark resonance frequency made reconstruction of the difference signal possible. This signal, which represents the difference between the dark and light Lorentz resonance curves, was detd. at various times during the decay. Anal. of these signals provided the time dependence for the changes in the real and imaginary parts of the dielec. const., which correspond to the densities of the trapped and free electrons. The decays of these parameters were characterized by three time domains. At the shortest times, the two parameters did not have the same time dependence. At intermediate times, the densities of both the trapped and free electrons had the same time dependence characterized by a power law decay, and a mechanism consistent with these results involves rapid equilibration between the free electrons and those in the shallow traps. Decay in this region was consistent with a dispersive transport mechanism. Intensity effects indicate satn. of the shallow traps. The 3rd region occurred at the break in the power law dependence indicating a bimol. recombination process. Measurements at higher temps. indicate a change from a bimol. to a monomol. recombination mechanism.
- 54Parkinson, P.; Joyce, H. J.; Gao, Q.; Tan, H. H.; Zhang, X.; Zou, J.; Jagadish, C.; Herz, L. M.; Johnston, M. B. Carrier Lifetime and Mobility Enhancement in Nearly Defect-free Core-shell Nanowires Measured Using Time-resolved Terahertz Spectroscopy Nano Lett. 2009, 9, 3349– 3353Google Scholar54https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXptVCks7Y%253D&md5=dfce7044b0801cc0282698055646ce6eCarrier lifetime and mobility enhancement in nearly defect-free core-shell nanowires measured using time-resolved terahertz spectroscopyParkinson, Patrick; Joyce, Hannah J.; Gao, Qiang; Tan, Hark Hoe; Zhang, Xin; Zou, Jin; Jagadish, Chennupati; Herz, Laura M.; Johnston, Michael B.Nano Letters (2009), 9 (9), 3349-3353CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)We have used transient terahertz photocond. measurements to assess the efficacy of 2-temp. growth and core-shell encapsulation techniques on the electronic properties of GaAs nanowires. 2-Temp. growth of the GaAs core leads to an almost doubling in charge-carrier mobility and a tripling of carrier lifetime. In addn., overcoating the GaAs core with a larger-bandgap material is shown to reduce the d. of surface traps by 82%, thereby enhancing the charge cond.
- 55Joyce, H. J.; Wong-Leung, J.; Yong, C.; Docherty, C. J.; Paiman, S.; Gao, Q.; Tan, H. H.; Jagadish, C.; Lloyd-Hughes, J.; Herz, L. M.et al. Ultra-low Surface Recombination Velocity in InP Nanowires Probed by Terahertz Spectroscopy Nano Lett. 2012, 12, 5325– 5330Google ScholarThere is no corresponding record for this reference.
- 56Joyce, H. J.; Docherty, C. J.; Gao, Q.; Tan, H. H.; Jagadish, C.; Lloyd-Hughes, J.; Herz, L. M.; Johnston, M. B. Electronic Properties of GaAs, Inas and InP Nanowires Studied by Terahertz Spectroscopy Nanotechnology 2013, 24, 214006Google Scholar56https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXht1ajt7zP&md5=147d13a2eeb16bdd25555aae6f47e6afElectronic properties of GaAs, InAs and InP nanowires studied by terahertz spectroscopyJoyce, Hannah J.; Docherty, Callum J.; Gao, Qiang; Tan, H. Hoe; Jagadish, Chennupati; Lloyd-Hughes, James; Herz, Laura M.; Johnston, Michael B.Nanotechnology (2013), 24 (21), 214006/1-214006/7, 7 pp.CODEN: NNOTER; ISSN:1361-6528. (IOP Publishing Ltd.)We have performed a comparative study of ultrafast charge carrier dynamics in a range of III-V nanowires using optical pump-terahertz probe spectroscopy. This versatile technique allows measurement of important parameters for device applications, including carrier lifetimes, surface recombination velocities, carrier mobilities and donor doping levels. GaAs, InAs and InP nanowires of varying diams. were measured. For all samples, the electronic response was dominated by a pronounced surface plasmon mode. Of the three nanowire materials, InAs nanowires exhibited the highest electron mobilities of 6000 cm2 V-1 s-1, which highlights their potential for high mobility applications, such as field effect transistors. InP nanowires exhibited the longest carrier lifetimes and the lowest surface recombination velocity of 170 cm s-1. This very low surface recombination velocity makes InP nanowires suitable for applications where carrier lifetime is crucial, such as in photovoltaics. In contrast, the carrier lifetimes in GaAs nanowires were extremely short, of the order of picoseconds, due to the high surface recombination velocity, which was measured as 5.4 × 105 cm s-1. These findings will assist in the choice of nanowires for different applications, and identify the challenges in producing nanowires suitable for future electronic and optoelectronic devices.
- 57Fortin, E.; Raga, F. Excitons in Molybdenum-disulfide Phys. Rev. B: Solid State 1975, 11, 905– 912Google Scholar57https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaE2MXhtlWntbo%253D&md5=f139eb8016fd2fe2b2e68b0cc235e5bfExcitons in molybdenum disulfideFortin, Emery; Raga, FrancescoPhysical Review B: Solid State (1975), 11 (2), 905-12CODEN: PLRBAQ; ISSN:0556-2805.Optical spectra of MoS2 in the excitonic region were obtained by photocond., photovoltaic effect and wavelength-modulated reflectivity at temps. of 300 to 4.2°K and in magnetic fields of up to 70 kG. Cleaved natural crystals and synthetic crystals studied in both the Faraday and Voigt configurations showed nearly the same characteristics. The results indicate the possibility of up to 4 excitonic series in MoS2 as in other Mo dichalcogenides, and support band-structure calcns. predicting flat conduction and valence bands originating from Mo orbitals. A ground-state anomaly of the A exciton is explained by a central cell correction of the type used by G. Harbeke and E. Tosatti (1972) to explain a similar anomaly in PbI2. The reduced mass of the A exciton is μ* ≃ 0.4m0, both from the study of the series itself, and from its magnetooptical properties, thus removing a large discrepancy found in previous work.
- 58Yong, C. K.; Joyce, H. J.; Lloyd-Hughes, J.; Gao, Q.; Tan, H. H.; Jagadish, C.; Johnston, M. B.; Herz, L. M. Ultrafast Dynamics of Exciton Formation In Semiconductor Nanowires Small 2012, 8, 1725– 1731Google ScholarThere is no corresponding record for this reference.
- 59Docherty, C. J.; Lin, C.; Joyce, H. J.; Nicholas, R. J.; Herz, L. M.; Li, L.; Johnston, M. B. Extreme Sensitivity of Graphene Photoconductivity To Environmental Gases Nat. Commun. 2012, 3, 1228Google Scholar59https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BC3s7mvFajuw%253D%253D&md5=98eaf4df586f605dd39c7b69749a7cf3Extreme sensitivity of graphene photoconductivity to environmental gasesDocherty Callum J; Lin Cheng-Te; Joyce Hannah J; Nicholas Robin J; Herz Laura M; Li Lain-Jong; Johnston Michael BNature communications (2012), 3 (), 1228 ISSN:.Graphene is a single layer of covalently bonded carbon atoms, which was discovered only 8 years ago and yet has already attracted intense research and commercial interest. Initial research focused on its remarkable electronic properties, such as the observation of massless Dirac fermions and the half-integer quantum Hall effect. Now graphene is finding application in touch-screen displays, as channels in high-frequency transistors and in graphene-based integrated circuits. The potential for using the unique properties of graphene in terahertz-frequency electronics is particularly exciting; however, initial experiments probing the terahertz-frequency response of graphene are only just emerging. Here we show that the photoconductivity of graphene at terahertz frequencies is dramatically altered by the adsorption of atmospheric gases, such as nitrogen and oxygen. Furthermore, we observe the signature of terahertz stimulated emission from gas-adsorbed graphene. Our findings highlight the importance of environmental conditions on the design and fabrication of high-speed, graphene-based devices.
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(35)
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(41)
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(37)
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(16)
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(1)
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(41)
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(20)
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(18)
, 7692-7701. https://doi.org/10.1021/acs.jpclett.0c02425
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(5)
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(5)
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(4)
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(3)
, 2026-2033. https://doi.org/10.1021/acs.nanolett.9b05344
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(2)
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(1)
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(50)
, 30676-30683. https://doi.org/10.1021/acs.jpcc.9b08483
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(9)
, 6078-6086. https://doi.org/10.1021/acs.nanolett.9b02005
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(13)
, 3763-3772. https://doi.org/10.1021/acs.jpclett.9b01422
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(16)
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- Lin Wang, Ce Xu, Ming-Yang Li, Lain-Jong Li, Zhi-Heng Loh. Unraveling Spatially Heterogeneous Ultrafast Carrier Dynamics of Single-Layer WSe2 by Femtosecond Time-Resolved Photoemission Electron Microscopy. Nano Letters 2018, 18
(8)
, 5172-5178. https://doi.org/10.1021/acs.nanolett.8b02103
- Renhao Dong, Tao Zhang, Xinliang Feng. Interface-Assisted Synthesis of 2D Materials: Trend and Challenges. Chemical Reviews 2018, 118
(13)
, 6189-6235. https://doi.org/10.1021/acs.chemrev.8b00056
- Yuanyuan Huang, Lipeng Zhu, Zehan Yao, Longhui Zhang, Chuan He, Qiyi Zhao, Jintao Bai, and Xinlong Xu . Terahertz Surface Emission from Layered MoS2 Crystal: Competition between Surface Optical Rectification and Surface Photocurrent Surge. The Journal of Physical Chemistry C 2018, 122
(1)
, 481-488. https://doi.org/10.1021/acs.jpcc.7b09723
- Yifei Yu, Guoqing Li, Lujun Huang, Andrew Barrette, Yong-Qing Cai, Yiling Yu, Kenan Gundogdu, Yong-Wei Zhang, and Linyou Cao . Enhancing Multifunctionalities of Transition-Metal Dichalcogenide Monolayers via Cation Intercalation. ACS Nano 2017, 11
(9)
, 9390-9396. https://doi.org/10.1021/acsnano.7b04880
- Xiao Xing, Litao Zhao, Zeyu Zhang, Xiankuan Liu, Kailin Zhang, Yang Yu, Xian Lin, Hua Ying Chen, Jin Quan Chen, Zuanming Jin, Jianhua Xu, and Guo-hong Ma . Role of Photoinduced Exciton in the Transient Terahertz Conductivity of Few-Layer WS2 Laminate. The Journal of Physical Chemistry C 2017, 121
(37)
, 20451-20457. https://doi.org/10.1021/acs.jpcc.7b05345
- Chaoliang Tan, Xiehong Cao, Xue-Jun Wu, Qiyuan He, Jian Yang, Xiao Zhang, Junze Chen, Wei Zhao, Shikui Han, Gwang-Hyeon Nam, Melinda Sindoro, and Hua Zhang . Recent Advances in Ultrathin Two-Dimensional Nanomaterials. Chemical Reviews 2017, 117
(9)
, 6225-6331. https://doi.org/10.1021/acs.chemrev.6b00558
- Yuanyuan Huang, Lipeng Zhu, Qiyi Zhao, Yaohui Guo, Zhaoyu Ren, Jintao Bai, and Xinlong Xu . Surface Optical Rectification from Layered MoS2 Crystal by THz Time-Domain Surface Emission Spectroscopy. ACS Applied Materials & Interfaces 2017, 9
(5)
, 4956-4965. https://doi.org/10.1021/acsami.6b13961
- Demetra Tsokkou, Xiaoyun Yu, Kevin Sivula, and Natalie Banerji . The Role of Excitons and Free Charges in the Excited-State Dynamics of Solution-Processed Few-Layer MoS2 Nanoflakes. The Journal of Physical Chemistry C 2016, 120
(40)
, 23286-23292. https://doi.org/10.1021/acs.jpcc.6b09267
- Paul D. Cunningham, Kathleen M. McCreary, Aubrey T. Hanbicki, Marc Currie, Berend T. Jonker, and L. Michael Hayden . Charge Trapping and Exciton Dynamics in Large-Area CVD Grown MoS2. The Journal of Physical Chemistry C 2016, 120
(10)
, 5819-5826. https://doi.org/10.1021/acs.jpcc.6b00647
- Chad Lunceford, Emanuel Borcean, and Jeff Drucker . Uniform and Repeatable Cold-Wall Chemical Vapor Deposition Synthesis of Single-Layer MoS2. Crystal Growth & Design 2016, 16
(2)
, 988-995. https://doi.org/10.1021/acs.cgd.5b01540
- Srabani Kar, Y. Su, R. R. Nair, and A. K. Sood . Probing Photoexcited Carriers in a Few-Layer MoS2 Laminate by Time-Resolved Optical Pump–Terahertz Probe Spectroscopy. ACS Nano 2015, 9
(12)
, 12004-12010. https://doi.org/10.1021/acsnano.5b04804
- Kyle J. Czech, Blaise J. Thompson, Schuyler Kain, Qi Ding, Melinda J. Shearer, Robert J. Hamers, Song Jin, and John C. Wright . Measurement of Ultrafast Excitonic Dynamics of Few-Layer MoS2 Using State-Selective Coherent Multidimensional Spectroscopy. ACS Nano 2015, 9
(12)
, 12146-12157. https://doi.org/10.1021/acsnano.5b05198
- Haining Wang, Changjian Zhang, and Farhan Rana . Surface Recombination Limited Lifetimes of Photoexcited Carriers in Few-Layer Transition Metal Dichalcogenide MoS2. Nano Letters 2015, 15
(12)
, 8204-8210. https://doi.org/10.1021/acs.nanolett.5b03708
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(46)
, 25703-25718. https://doi.org/10.1021/acs.jpcc.5b07432
- Hyun Jeong, Seungho Bang, Hye Min Oh, Hyeon Jun Jeong, Sung-Jin An, Gang Hee Han, Hyun Kim, Ki Kang Kim, Jin Cheol Park, Young Hee Lee, Gilles Lerondel, and Mun Seok Jeong . Semiconductor–Insulator–Semiconductor Diode Consisting of Monolayer MoS2, h-BN, and GaN Heterostructure. ACS Nano 2015, 9
(10)
, 10032-10038. https://doi.org/10.1021/acsnano.5b04233
- Zhaogang Nie, Run Long, Jefri S. Teguh, Chung-Che Huang, Daniel W. Hewak, Edwin K. L. Yeow, Zexiang Shen, Oleg V. Prezhdo, and Zhi-Heng Loh . Ultrafast Electron and Hole Relaxation Pathways in Few-Layer MoS2. The Journal of Physical Chemistry C 2015, 119
(35)
, 20698-20708. https://doi.org/10.1021/acs.jpcc.5b05048
- Joohoon Kang, Joshua D. Wood, Spencer A. Wells, Jae-Hyeok Lee, Xiaolong Liu, Kan-Sheng Chen, and Mark C. Hersam . Solvent Exfoliation of Electronic-Grade, Two-Dimensional Black Phosphorus. ACS Nano 2015, 9
(4)
, 3596-3604. https://doi.org/10.1021/acsnano.5b01143
- Haining Wang, Changjian Zhang, and Farhan Rana . Ultrafast Dynamics of Defect-Assisted Electron–Hole Recombination in Monolayer MoS2. Nano Letters 2015, 15
(1)
, 339-345. https://doi.org/10.1021/nl503636c
- Saloni Sharma, Divyansh Pratap Singh, Shreeya Rane, Jishnu Murukeshan, Tymofii S. Pieshkov, Shubhda Srivastava, Soumyabrata Roy, Anand B. Puthirath, Guanhui Gao, Ruturaj Puranik, Utkarsh Pandey, Snehal Haldankar, Vibhavari Parkar, Fatimath Faseela, S. S. Prabhu, Dibakar Roychowdhury, Pulickel M. Ajayan, Bipin Kumar Gupta. Temperature dependent trion induced tunability of terahertz signal response enabled by intraband transitions in monolayer WS2. Journal of Materials Chemistry C 2025, https://doi.org/10.1039/D4TC04218B
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(28)
https://doi.org/10.1002/advs.202402615
- Umberto Celano, Daniel Schmidt, Carlos Beitia, George Orji, Albert V. Davydov, Yaw Obeng. Metrology for 2D materials: a perspective review from the international roadmap for devices and systems. Nanoscale Advances 2024, 6
(9)
, 2260-2269. https://doi.org/10.1039/D3NA01148H
- Enen Li, Jincheng Wei, Tianyu Zhang, Hujie Wan, Yuguang Cheng, Jiafeng Xie, Hong Li, Kai Zhang, Jingyin Xu, Jinkang Hu, Qiye Wen, Xu Xiao, Tao Zhao, Min Hu, Fuhai Su, Tianwu Wang, Guangyou Fang. Charge Carriers Localization Effect Revealed through Terahertz Spectroscopy of MXene: Ti
3
C
2
T
x
. Small 2024, 20
(16)
https://doi.org/10.1002/smll.202306200
- Eleonora Pavoni, Elaheh Mohebbi, Gian Marco Zampa, Pierluigi Stipa, Luca Pierantoni, Emiliano Laudadio, Davide Mencarelli. First principles study of WSe
2
and the effect of V doping on the optical and electronic properties. Materials Advances 2024, 5
(6)
, 2230-2237. https://doi.org/10.1039/D3MA00940H
- Satoshi Kusaba, Haw-Wei Lin, Ryo Tamaki, Ikufumi Katayama, Jun Takeda, Geoffrey A. Blake. Terahertz sum-frequency excitation of coherent optical phonons in the two-dimensional semiconductor WSe2. Applied Physics Letters 2024, 124
(12)
https://doi.org/10.1063/5.0191558
- Qiong Wu, Qiangwei Yin, Sijie Zhang, Tianchen Hu, Dong Wu, Li Yue, Bohan Li, Shuxiang Xu, Rongsheng Li, Qiaomei Liu, Hechang Lei, Tao Dong, Nanlin Wang. Pump‐Induced Terahertz Conductivity Response and Peculiar Bound State in Mn
3
Si
2
Te
6. Advanced Optical Materials 2024, 12
(9)
https://doi.org/10.1002/adom.202301863
- Taketo Handa, Madisen Holbrook, Nicholas Olsen, Luke N. Holtzman, Lucas Huber, Hai I. Wang, Mischa Bonn, Katayun Barmak, James C. Hone, Abhay N. Pasupathy, Xiaoyang Zhu. Spontaneous exciton dissociation in transition metal dichalcogenide monolayers. Science Advances 2024, 10
(5)
https://doi.org/10.1126/sciadv.adj4060
- Partha Pratim Bag, Dev Kumar Thapa, Govind Pratap Singh, Arnab Maity, Anup Gurung. Exploring modern developments in diverse 2D photocatalysts for water oxidation. Journal of Porous Materials 2024, 31
(1)
, 1-32. https://doi.org/10.1007/s10934-023-01516-1
- Eugenio Cinquanta, Eva Arianna Aurelia Pogna, Lorenzo Gatto, Salvatore Stagira, Caterina Vozzi. Charge carrier dynamics in 2D materials probed by ultrafast THzspectroscopy. Advances in Physics: X 2023, 8
(1)
https://doi.org/10.1080/23746149.2022.2120416
- Saloni Sharma, Pooja Chauhan, Shreeya Rane, Utkarsh Raj, Shubhda Srivastava, Z. A. Ansari, Dibakar Roy Chowdhury, Bipin Kumar Gupta. New insights into APCVD grown monolayer MoS2 using time-domain terahertz spectroscopy. Scientific Reports 2023, 13
(1)
https://doi.org/10.1038/s41598-023-31102-z
- Neetesh Dhakar, Sandeep Kumar, Anand Nivedan, Sunil Kumar. An interplay between optical rectification and transient photocurrent effect on THz pulse generation from bulk MoS
2
layered crystal. Journal of Physics D: Applied Physics 2023, 56
(43)
, 435105. https://doi.org/10.1088/1361-6463/ace4d9
- S. Mitra, L. Avazpour, I. Knezevic. Terahertz conductivity of monolayer MoS$$_2$$. Journal of Computational Electronics 2023, 22
(5)
, 1319-1326. https://doi.org/10.1007/s10825-023-02023-x
- Abdullah Alharbi, Naif Alshamrani, Hadba Hussain, Mohammed Alhamdan, Salman Alfihed. Two-Dimensional Materials for Terahertz Emission. 2023https://doi.org/10.5772/intechopen.110878
- Enen Li, Jin Yang, Kai Zhang, Hong Li, Ying Xu, Fuhai Su, Tianwu Wang, Guangyou Fang. Systematic Study of Two-Color Air Plasma Broadband THz-TDS. IEEE Transactions on Terahertz Science and Technology 2023, 13
(5)
, 476-484. https://doi.org/10.1109/TTHZ.2023.3292547
- Eswari Elango, Veera Prabu Kannan, Sridharan Madangurusamy, Rakesh Kumar Karn, Devasish Chowdhury, Chandan Upadhyay, Asha Yadav. Magnetron-Sputtered Silver Nanoparticles for Surface Plasmons for Terahertz Sensors. Journal of Electronic Materials 2023, 52
(7)
, 4289-4294. https://doi.org/10.1007/s11664-023-10211-5
- Stan E. T. ter Huurne, Niels J. J. van Hoof, Jaime Gómez Rivas. Thickness-dependent Auger scattering in a single WS
2
microcrystal probed with time-resolved terahertz near-field microscopy. Optics Letters 2023, 48
(3)
, 708. https://doi.org/10.1364/OL.477389
- Yuze Hu, Mingyu Tong, Siyang Hu, Weibao He, Xiang'ai Cheng, Tian Jiang. Reassessing Fano Resonance for Broadband, High‐Efficiency, and Ultrafast Terahertz Wave Switching. Advanced Science 2023, 10
(2)
https://doi.org/10.1002/advs.202204494
- Fulin Zhuo, Jie Wu, Binhong Li, Moyang Li, Chee Leong Tan, Zhongzhong Luo, Huabin Sun, Yong Xu, Zhihao Yu. Modifying the Power and Performance of 2-Dimensional MoS
2
Field Effect Transistors. Research 2023, 6 https://doi.org/10.34133/research.0057
- 张泽亮 Zhang Zeliang, 齐鹏飞 Qi Pengfei, 郭兰军 Guo Lanjun, 张楠 Zhang Nan, 林列 Lin Lie, 刘伟伟 Liu Weiwei. 太赫兹超分辨近场成像方法研究综述. Acta Optica Sinica 2023, 43
(6)
, 0600001. https://doi.org/10.3788/AOS221632
- Yikai Zheng, Harikrishnan Ravichandran, Thomas F. Schranghamer, Nicholas Trainor, Joan M. Redwing, Saptarshi Das. Hardware implementation of Bayesian network based on two-dimensional memtransistors. Nature Communications 2022, 13
(1)
https://doi.org/10.1038/s41467-022-33053-x
- Xingfang Luo, Peiwen Xiang, Heming Yu, Shan Huang, Ting Yu, Yuan-Feng Zhu. Terahertz Metamaterials Broadband Perfect Absorber Based on Molybdenum Disulfide. IEEE Photonics Technology Letters 2022, 34
(20)
, 1100-1103. https://doi.org/10.1109/LPT.2022.3202991
- Jiwon Jung, Jae Whan Park, Jaeyoung Kim, Han Woong Yeom. Surface enhanced electron correlation on the trivial quasi-two-dimensional bulk insulator
1
T
−
TaS
2
. Physical Review B 2022, 106
(15)
https://doi.org/10.1103/PhysRevB.106.155406
- Shuva Mitra, Irena Knezevic. Simulation of ac conductivity of monolayer MoS
2
at terahertz frequencies. 2022, 101-102. https://doi.org/10.1109/NUSOD54938.2022.9894759
- Changwon Seo, Teun-Teun Kim. Terahertz near-field spectroscopy for various applications. Journal of the Korean Physical Society 2022, 81
(6)
, 549-561. https://doi.org/10.1007/s40042-022-00404-2
- Raul Perea-Causin, Samuel Brem, Ermin Malic. Trion-phonon interaction in atomically thin semiconductors. Physical Review B 2022, 106
(11)
https://doi.org/10.1103/PhysRevB.106.115407
- Jing Chen, Fan Wu, Ping Li, Jianguo Hu, He Tian, Xiao-Ming Wu, Yi Yang, Tian-Ling Ren. The α-In
2
Se
3
THz Photodetector. IEEE Transactions on Electron Devices 2022, 69
(8)
, 4371-4376. https://doi.org/10.1109/TED.2022.3184621
- Xiao Xing, Zeyu Zhang, Chenjing Quan, Litao Zhao, Chunwei Wang, Tingyuan Jia, Junfeng Ren, Juan Du, Yuxin Leng. Tunable ultrafast electron transfer in WSe
2
–graphene heterostructures enabled by atomic stacking order. Nanoscale 2022, 14
(19)
, 7418-7425. https://doi.org/10.1039/D1NR07698A
- Qiao-Lu Lin, Zheng-Fang Qian, Xiang-Yu Dai, Yi-Ling Sun, Ren-Heng Wang. Regulation of electronic structure of monolayer MoS2 by pressure. Rare Metals 2022, 41
(5)
, 1761-1770. https://doi.org/10.1007/s12598-021-01888-w
- M.J. Ambrosio, E. Plesiat, P. Decleva, P.M. Echenique, R. Díez Muiño, F. Martín. Cluster approach to scattering in MoS2 photoemission. Chemical Physics 2022, 557 , 111476. https://doi.org/10.1016/j.chemphys.2022.111476
- Madhura Ghosh Dastidar, Immanuel Thekkooden, Pramoda K. Nayak, Vidya Praveen Bhallamudi. Quantum emitters and detectors based on 2D van der Waals materials. Nanoscale 2022, 14
(14)
, 5289-5313. https://doi.org/10.1039/D1NR08193D
- Artur P. Herman, Szymon J. Zelewski, Kamil Misztal, Robert Kudrawiec. Probing the long-lived photo-generated charge carriers in transition metal dichalcogenides by time-resolved microwave photoconductivity. Nanophotonics 2022, 11
(7)
, 1335-1344. https://doi.org/10.1515/nanoph-2021-0741
- Qirui Liu, Ke Wei, Yuxiang Tang, Zhongjie Xu, Xiang'ai Cheng, Tian Jiang. Visualizing Hot‐Carrier Expansion and Cascaded Transport in WS
2
by Ultrafast Transient Absorption Microscopy. Advanced Science 2022, 9
(10)
https://doi.org/10.1002/advs.202105746
- S. Kar, J. Lake, S.O. Adeyemo, T.S. Santra, H.J. Joyce. The physics of terahertz negative photoconductivity in low-dimensional materials. Materials Today Physics 2022, 23 , 100631. https://doi.org/10.1016/j.mtphys.2022.100631
- Xiongzhi Zeng, Wei Hu, Xiao Zheng, Jin Zhao, Zhenyu Li, Jinlong Yang. Computational characterization of nanosystems. Chinese Journal of Chemical Physics 2022, 35
(1)
, 1-15. https://doi.org/10.1063/1674-0068/cjcp2111233
- Govindan Kutty Rajendran Nair, Zhaowei Zhang, Fuchen Hou, Ali Abdelaziem, Xiaodong Xu, Steve Wu Qing Yang, Nan Zhang, Weiqi Li, Chao Zhu, Yao Wu, Heng Weiling, Lixing Kang, Teddy Salim, Jiadong Zhou, Lin Ke, Junhao Lin, Xingji Li, Weibo Gao, Zheng Liu. Phase-pure two-dimensional FexGeTe2 magnets with near-room-temperature TC. Nano Research 2022, 15
(1)
, 457-464. https://doi.org/10.1007/s12274-021-3502-0
- Abdoalbaset Abohmra, Zia Ullah Khan, Hasan T. Abbas, Nosherwan Shoaib, Muhammad A. Imran, Qammer H. Abbasi. Two-Dimensional Materials for Future Terahertz Wireless Communications. IEEE Open Journal of Antennas and Propagation 2022, 3 , 217-228. https://doi.org/10.1109/OJAP.2022.3143994
- Alka Jakhar, Prabhat Kumar, Sajid Husain, Veerendra Dhyani, Abhilasha Chouksey, Prashant Kumar Rai, J S Rawat, Samaresh Das. Bilayer MoS
2
on silicon for higher terahertz amplitude modulation. Nano Express 2021, 2
(4)
, 040004. https://doi.org/10.1088/2632-959X/ac1ef6
- Basant Chitara, Kunyan Zhang, Martha Y. Garcia Cervantes, Tej B. Limbu, Bikram Adhikari, Shengxi Huang, Fei Yan. Probing charge transfer in 2D MoS2/tellurene type-II p–n heterojunctions. MRS Communications 2021, 11
(6)
, 868-872. https://doi.org/10.1557/s43579-021-00117-w
- Ziming Wang, Jie Qiao, Shuqi Zhao, Shilei Wang, Chuan He, Xutang Tao, Shanpeng Wang. Recent progress in terahertz modulation using photonic structures based on two‐dimensional materials. InfoMat 2021, 3
(10)
, 1110-1133. https://doi.org/10.1002/inf2.12236
- Sunil Kumar, Arvind Singh, Anand Nivedan, Sandeep Kumar, Seok Joon Yun, Young Hee Lee, Marc Tondusson, Jérôme Degert, Jean Oberle, Eric Freysz. Sub‐bandgap activated charges transfer in a graphene‐MoS
2
‐graphene heterostructure. Nano Select 2021, 2
(10)
, 2019-2028. https://doi.org/10.1002/nano.202000159
- Shuo Liu, Rui Ma, Yuanwei Li, Linwan Zhao, Yuanqin Xia, Xiaolong Dong, Yajun Pang. D-shaped surface plasmon resonance biosensor based on
MoS
2
in terahertz band. Optical Fiber Technology 2021, 66 , 102631. https://doi.org/10.1016/j.yofte.2021.102631
- Zexin Li, Dongyan Li, Haoyun Wang, Ping Chen, Lejing Pi, Xing Zhou, Tianyou Zhai. Intercalation Strategy in 2D Materials for Electronics and Optoelectronics. Small Methods 2021, 5
(9)
https://doi.org/10.1002/smtd.202100567
- Stan ter Huurne, Niels van Hoof, Rasmus Godiksen, Sara ElRafey, Alberto G. Curto, Jaime Gomez Rivas. THz microscopy on a single WS
2
microcrystal. 2021, 1-2. https://doi.org/10.1109/IRMMW-THz50926.2021.9567372
- Haoxuan Jiao, Xulei Qin, Ye Li, Guozheng Wang. Refractive index and optical-pump THz-probe measurement of ReSe2. Infrared Physics & Technology 2021, 116 , 103752. https://doi.org/10.1016/j.infrared.2021.103752
- Berardi Sensale‐Rodriguez. Advanced Devices Using Two‐Dimensional Layer Technology. 2021, 251-284. https://doi.org/10.1002/9781119460749.ch7
- Young‐Mi Bahk, Minah Seo. Ultrasensitive Terahertz Nano‐Probing for Semiconductors Using Nanogap Structures. 2021, 1-15. https://doi.org/10.1002/3527600434.eap839
- Zheng Liu, Shujuan Xu, Binhe Xie, Yuanyuan Luo, Hongying Mei, Huachao Jiang, Zhi Zeng, Guangtao Fei, Fuhai Su. Ultrafast dynamics of photoconductivity in lead sulfide nanocrystals in terahertz region. Journal of Alloys and Compounds 2021, 867 , 158873. https://doi.org/10.1016/j.jallcom.2021.158873
- Dana B. Sulas-Kern, Hanyu Zhang, Zhaodong Li, Jeffrey L. Blackburn. Interplay between microstructure, defect states, and mobile charge generation in transition metal dichalcogenide heterojunctions. Nanoscale 2021, 13
(17)
, 8188-8198. https://doi.org/10.1039/D1NR00384D
- Qi Song, Lu Chai, Junqi Chen, Weining Liu, Qing Ma, Yanfeng Li, Minglie Hu. Optically Tuned Wide-Band Terahertz Modulation, Charge Carrier Dynamics and Photoconductivity of Femtosecond Laser Ablated Titanium Disulfide Nanosheet Devices. IEEE Journal of Selected Topics in Quantum Electronics 2021, 27
(3)
, 1-6. https://doi.org/10.1109/JSTQE.2020.2996248
- Lei Gao, Zhenliang Hu, Junpeng Lu, Hongwei Liu, Zhenhua Ni. Defect-related dynamics of photoexcited carriers in 2D transition metal dichalcogenides. Physical Chemistry Chemical Physics 2021, 23
(14)
, 8222-8235. https://doi.org/10.1039/D1CP00006C
- I Nevinskas, R Norkus, A Geižutis, L Kulyuk, A Miku, K Sushkevich, A Krotkus. Terahertz pulse emission from photoexcited bulk crystals of transition metal dichalcogenides. Journal of Physics D: Applied Physics 2021, 54
(11)
, 115105. https://doi.org/10.1088/1361-6463/abcc26
- Shuai Fu, Indy du Fossé, Xiaoyu Jia, Jingyin Xu, Xiaoqing Yu, Heng Zhang, Wenhao Zheng, Sven Krasel, Zongping Chen, Zhiming M. Wang, Klaas-Jan Tielrooij, Mischa Bonn, Arjan J. Houtepen, Hai I. Wang. Long-lived charge separation following pump-wavelength–dependent ultrafast charge transfer in graphene/WS
2
heterostructures. Science Advances 2021, 7
(9)
https://doi.org/10.1126/sciadv.abd9061
- Martin Mittendorff, Stephan Winnerl, Thomas E. Murphy. 2D THz Optoelectronics. Advanced Optical Materials 2021, 9
(3)
https://doi.org/10.1002/adom.202001500
- A. V. Gorbatova, D. I. Khusyainov, A. M. Buryakov. Generation of the THz radiation in Mo0.5W0.5S2 solid solution. 2021, 020009. https://doi.org/10.1063/5.0055454
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Abstract
Figure 1
Figure 1. Characterization of monolayer MoS2 (a–d), trilayer MoS2 (e–h), and monolayer WSe2 samples. (a), (e), (i) Schematic representation of the samples. (b), (f), (j) Raman spectroscopy of the samples. The splitting between the E2G1 and A1G peaks is indicative of layer number. (c), (g), (k) AFM images of the samples. Height profiles are overlaid at the position they were taken. The step heights again confirm the layer number identification. (d), (h), (l) Optical transmission spectrum of the samples. Absorption features associated with the “A” and “B” excitons are labeled.
Figure 2
Figure 2. Photodynamics of monolayer MoS2 measured by optical pump–THz probe spectroscopy and PL upconversion spectroscopy. (a) The blue circles show the normalized THz photoconductivity, ΔσTHz, of monolayer MoS2 as a function of time after photoexcitation by 3.1 eV photons (absorbed fluence ∼7 × 1013 cm–2/pulse). The black crosses show the normalized PL emitted at 1.86 eV as a function of time after photoexcitation by 3 eV photons (absorbed fluence ∼6.9 × 1012 cm–2/pulse). Solid lines are biexponential fits to the data. (b) Similar ΔσTHz (blue circles) and PL (black crosses) measurements following photoexcitation on resonance with the direct gap excitons (1.9 eV photons with absorbed fluence ∼1.2 × 1014 cm–2 for ΔσTHz, and 2.1 eV photons with absorbed fluence ∼3.5 × 1012 cm–2 for PL).
Figure 3
Figure 3. Normalized photodynamics of monolayer WSe2 measured by optical pump-THz probe spectroscopy. The blue crosses show the THz photoconductivity of WSe2, ΔσTHz, as a function of time after photoexcitation by 35 fs pulses of above-gap photons with energy of 3.1 eV. The absorbed photon fluence was ∼4.2 × 1013 cm–2/pulse. The red squares show ΔσTHz of WSe2 photoexcited resonantly with the “A” exciton at a photon energy of 1.65 eV. The fluence absorbed in the WSe2 layer was ∼3.3 × 1013 cm–2/pulse. Solid lines are biexponential fits to the data.
Figure 4
Figure 4. Comparison of THz photoconductivity, ΔσTHz, in trilayer (green crosses) and monolayer (blue squares) MoS2 as a function of time after photoexcitation by 175 μJ cm–2 35 fs pulses of 3.1 eV photons. The absorbed fluence of photons were ∼1.4 × 1014 cm–2/pulse in the trilayer sample and ∼7 × 1013 cm–2/pulse in the monolayer sample. Solid lines show biexponential fits to the data. Fit parameters are listed in Supporting Information Table 1. Additional reflections due to the thinness of the substrate have been removed from the trilayer data for clarity (see Supporting Information).
Figure 5
Figure 5. THz complex photoconductivity spectra, Δσ(ν). (a) Monolayer MoS2 1.5 ps after photoexcitation with 3 eV (squares) and 1.9 eV (crosses) photons. A low energy resonance at ∼18 meV (4.5 THz) is apparent. (b) Trilayer MoS2, 1.5 ps after photoexcitation by 1.9 eV photons. (c) Monolayer WSe2, 1 ps after photoexcitation on resonance with the WSe2 “A” exciton at 1.65 eV. Solid lines show possible fits to the data, detailed in the Supporting Information.
References
This article references 59 other publications.
- 1Smith, F. W.; Le, H. Q.; Diadiuk, V.; Hollis, M. A.; Calawa, A. R.; Gupta, S.; Frankel, M.; Dykaar, D. R.; Mourou, G. A.; Hsiang, T. Y. Picosecond GaAs-based Photoconductive Optoelectronic Detectors Appl. Phys. Lett. 1989, 54, 890– 892There is no corresponding record for this reference.
- 2Nolte, D. D. Semi-insulating Semiconductor Heterostructures: Optoelectronic Properties and Applications J. Appl. Phys. 1999, 85, 6259– 62892https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK1MXisFCiur8%253D&md5=faaad20367c407efdfda885b9522c0cbSemi-insulating semiconductor heterostructures: Optoelectronic properties and applicationsNolte, David D.Journal of Applied Physics (1999), 85 (9), 6259-6289CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)This review with 279 refs. covers a spectrum of optoelectronic properties of and uses for semi-insulating semiconductor heterostructures and thin films, including epilayers and quantum wells. Compensation by doping, implantation, and nonstoichiometric growth are described in terms of the properties of point defects and Fermi level stabilization and pinning. The principal optical and optoelectronic properties of semi-insulating epilayers and heterostructures, such as excitonic electroabsorption of quantum-confined excitons, are described, in addn. to optical absorption by metallic or semimetallic ppts. in these layers. Low-temp. grown quantum wells that have an As-rich nonstoichiometry and a supersatd. concn. of grown-in vacancies are discussed. These heterostructures experience transient enhanced diffusion and superlattice disordering. The review discusses the performance of optoelectronic heterostructures and microcavities that contain semi-insulating layers, such as buried heterostructure stripe lasers, vertical cavity surface emitting lasers, and optical electroabsorption modulators. Short time-scale applications arise from the ultrashort carrier lifetimes in semi-insulating materials, such as in photoconductors for terahertz generation, and in saturable absorbers for mode-locking solid-state lasers. This review also comprehensively describes the properties and applications of photorefractive heterostructures. The low dark-carrier concns. of semi-insulating heterostructures make these materials highly sensitive as dynamic holog. thin films that are useful for adaptive optics applications. The high mobilities of free carriers in photorefractive heterostructures produce fast dielec. relaxation rates that allow light-induced space-charge gratings to adapt to rapidly varying optical fringe patterns, canceling out environmental noise during interferometric detection in laser-based ultrasound, and in optical coherence tomog. They are also the functional layers in high-sensitivity dynamic holog. materials that replace static holograms in Fourier imaging systems and in exptl. Tbit/s optical systems. Semi-insulating heterostructures and their applications have attained a degree of maturity, but many crit. materials science issues remain unexplored.
- 3Krotkus, A. Semiconductors for Terahertz Photonics Applications J. Phys. D: Appl. Phys. 2010, 43, 2730013https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXpsVOltrw%253D&md5=70a0b88ad1d1dada87c45ffecdd8d41cSemiconductors for terahertz photonics applicationsKrotkus, ArunasJournal of Physics D: Applied Physics (2010), 43 (27), 273001/1-273001/21CODEN: JPAPBE; ISSN:0022-3727. (Institute of Physics Publishing)A review. Generation and measurement of ultrashort, subpicosecond pulses of electromagnetic radiation with their characteristic Fourier spectra that reach far into terahertz (THz) frequency range has recently become a versatile tool of far-IR spectroscopy and imaging. This technique, THz time-domain spectroscopy, in addn. to a femtosecond pulse laser, requires semiconductor components manufd. from materials with a short photoexcited carrier lifetime, high carrier mobility and large dark resistivity. Here we will review the most important developments in the field of investigation of such materials. The main characteristics of low-temp.-grown or ion-implanted GaAs and semiconducting compds. sensitive in the wavelength ranges around 1 μm and 1.5 μm will be surveyed. The second part of the paper is devoted to the effect of surface emission of THz transients from semiconductors illuminated by femtosecond laser pulses. The main phys. mechanisms leading to this emission as well as their manifestation in various crystals will be described.
- 4Loka, H. S.; Benjamin, S. D.; Smith, P. W. E. Optical Characterization of Low-temperature-grown GaAs for Ultrafast All-optical Switching Devices IEEE J. Quantum Electron. 1998, 34, 1426– 1437There is no corresponding record for this reference.
- 5Shen, Y. C.; Upadhya, P. C.; Linfield, E. H.; Beere, H. E.; Davies, A. G. Ultrabroadband Terahertz Radiation from Low-temperature-grown GaAs Photoconductive Emitters Appl. Phys. Lett. 2003, 83, 3117– 31195https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3sXotVarurs%253D&md5=39bee2eb97d6cb38fc9e2832bb97885cUltrabroadband terahertz radiation from low-temperature-grown GaAs photoconductive emittersShen, Y. C.; Upadhya, P. C.; Linfield, E. H.; Beere, H. E.; Davies, A. G.Applied Physics Letters (2003), 83 (15), 3117-3119CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)THz radiation was generated with a biased and asym. excited low-temp.-grown GaAs photoconductive emitter, and characterized with a 20-μm-thick ZnTe crystal using free-space electrooptic sampling. Using a backward collection scheme, THz radiation with frequency components >30 THz were obtained, the highest ever obsd. for photoconductive emitters. Spectra over ν = 0.3-20 THz are presented, demonstrating the use of this source for ultrabroadband THz spectroscopy.
- 6Ferguson, B.; Zhang, X.-C. Materials for Terahertz Science and Technology Nat. Mater. 2002, 1, 26– 336https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD38XntlWlsrc%253D&md5=39a00a51aea9eed67983a713682676a2Materials for terahertz science and technologyFerguson, Bradley; Zhang, Xi-ChengNature Materials (2002), 1 (1), 26-33CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)A review. Terahertz spectroscopy systems use far-IR radiation to ext. mol. spectral information in an otherwise inaccessible portion of the electromagnetic spectrum. Materials research is an essential component of modem terahertz systems: novel, higher-power terahertz sources rely heavily on new materials such as quantum cascade structures. At the same time, terahertz spectroscopy and imaging provide a powerful tool for the characterization of a broad range of materials, including semiconductors and biomols.
- 7Castro-Camus, E.; Lloyd-Hughes, J.; Fu, L.; Tan, H. H.; Jagadish, C.; Johnston, M. B. An Ion-implanted InP Receiver for Polarization Resolved Terahertz Spectroscopy Opt. Express 2007, 15, 7047– 7057There is no corresponding record for this reference.
- 8Mangeney, J. THz Photoconductive Antennas Made from Ion-bombarded Semiconductors J. Infrared, Millimeter, Terahertz Waves 2012, 33, 455– 473There is no corresponding record for this reference.
- 9Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Two-dimensional Atomic Crystals Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 10451– 104539https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2MXntVSit7g%253D&md5=1ce9e5f5eb0f7b9abb033d4a690d49c3Two-dimensional atomic crystalsNovoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K.Proceedings of the National Academy of Sciences of the United States of America (2005), 102 (30), 10451-10453CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)The authors report free-standing at. crystals that are strictly 2-dimensional and can be viewed as individual at. planes pulled out of bulk crystals or as unrolled single-wall nanotubes. By using micromech. cleavage, the authors prepd. and studied a variety of 2-dimensional crystals including single layers of boron nitride, graphite, several dichalcogenides, and complex oxides. These atomically thin sheets (essentially gigantic 2-dimensional mols. unprotected from the immediate environment) are stable under ambient conditions, exhibit high crystal quality, and are continuous on a macroscopic scale.
- 10Friend, R. H.; Yoffe, A. D. Electronic-properties of Intercalation Complexes of the Transition-metal Dichalcogenides Adv. Phys. 1987, 36, 1– 94There is no corresponding record for this reference.
- 11Wang, Q. H.; Kalantar-zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Electronics and Optoelectronics of Two-dimensional Transition Metal Dichalcogenides Nat. Nanotechnol. 2012, 7, 699– 71211https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38Xhs1ajtr7P&md5=4e45d586c6ac7b0676a461f61a53db68Electronics and optoelectronics of two-dimensional transition metal dichalcogenidesWang, Qing Hua; Kalantar-Zadeh, Kourosh; Kis, Andras; Coleman, Jonathan N.; Strano, Michael S.Nature Nanotechnology (2012), 7 (11), 699-712CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)A review. The remarkable properties of graphene have renewed interest in inorg., two-dimensional materials with unique electronic and optical attributes. Transition metal dichalcogenides (TMDCs) are layered materials with strong in-plane bonding and weak out-of-plane interactions enabling exfoliation into two-dimensional layers of single unit cell thickness. Although TMDCs were studied for decades, recent advances in nanoscale materials characterization and device fabrication have opened up new opportunities for two-dimensional layers of thin TMDCs in nanoelectronics and optoelectronics. TMDCs such as MoS2, MoSe2, WS2 and WSe2 have sizable bandgaps that change from indirect to direct in single layers, allowing applications such as transistors, photodetectors and electroluminescent devices. The authors review the historical development of TMDCs, methods for prepg. atomically thin layers, their electronic and optical properties, and prospects for future advances in electronics and optoelectronics.
- 12Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L. J.; Loh, K. P.; Zhang, H. The Chemistry of Two-dimensional Layered Transition Metal Dichalcogenide Nanosheets Nat. Chem. 2013, 5, 263– 27512https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BC3svotVamsA%253D%253D&md5=3dbc30db63b6a32231ecf5cae8a71cd9The chemistry of two-dimensional layered transition metal dichalcogenide nanosheetsChhowalla Manish; Shin Hyeon Suk; Eda Goki; Li Lain-Jong; Loh Kian Ping; Zhang HuaNature chemistry (2013), 5 (4), 263-75 ISSN:.Ultrathin two-dimensional nanosheets of layered transition metal dichalcogenides (TMDs) are fundamentally and technologically intriguing. In contrast to the graphene sheet, they are chemically versatile. Mono- or few-layered TMDs - obtained either through exfoliation of bulk materials or bottom-up syntheses - are direct-gap semiconductors whose bandgap energy, as well as carrier type (n- or p-type), varies between compounds depending on their composition, structure and dimensionality. In this Review, we describe how the tunable electronic structure of TMDs makes them attractive for a variety of applications. They have been investigated as chemically active electrocatalysts for hydrogen evolution and hydrosulfurization, as well as electrically active materials in opto-electronics. Their morphologies and properties are also useful for energy storage applications such as electrodes for Li-ion batteries and supercapacitors.
- 13Jariwala, D.; Sangwan, V. K.; Lauhon, L. J.; Marks, T. J.; Hersam, M. C. Emerging Device Applications for Semiconducting Two-dimensional Transition Metal Dichalcogenides ACS Nano 2014, 8, 1102– 112013https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhsVKlt78%253D&md5=185f8627d2582249825fe739fcee19efEmerging Device Applications for Semiconducting Two-Dimensional Transition Metal DichalcogenidesJariwala, Deep; Sangwan, Vinod K.; Lauhon, Lincoln J.; Marks, Tobin J.; Hersam, Mark C.ACS Nano (2014), 8 (2), 1102-1120CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)A review. With advances in exfoliation and synthetic techniques, atomically thin films of semiconducting transition metal dichalcogenides have recently been isolated and characterized. Their two-dimensional structure, coupled with a direct band gap in the visible portion of the electromagnetic spectrum, suggests suitability for digital electronics and optoelectronics. Toward that end, several classes of high-performance devices are reported along with significant progress in understanding their phys. properties. Here, the authors present a review of the architecture, operating principles, and physics of electronic and optoelectronic devices based on ultrathin transition metal dichalcogenide semiconductors. By critically assessing and comparing the performance of these devices with competing technologies, the merits and shortcomings of this emerging class of electronic materials are identified, thereby providing a road map for future development.
- 14Coehoorn, R.; Haas, C.; Degroot, R. A. Electronic-structure of MoSe2, MoS2, and WSe2. II The Nature of the Optical Band-gaps Phys. Rev. B: Condens. Matter Mater. Phys. 1987, 35, 6203– 6206There is no corresponding record for this reference.
- 15Kim, Y.; Huang, J. L.; Lieber, C. M. Characterization of Nanometer Scale Wear and Oxidation of Transition-metal Dichalcogenide Lubricants by Atomic Force Microscopy Appl. Phys. Lett. 1991, 59, 3404– 3406There is no corresponding record for this reference.
- 16Karunadasa, H. I.; Montalvo, E.; Sun, Y. J.; Majda, M.; Long, J. R.; Chang, C. J. A Molecular MoS2 Edge Site Mimic for Catalytic Hydrogen Generation Science 2012, 335, 698– 70216https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhvVCjsr8%253D&md5=66849a25acaecaf60e1922de5397b061A Molecular MoS2 Edge Site Mimic for Catalytic Hydrogen GenerationKarunadasa, Hemamala I.; Montalvo, Elizabeth; Sun, Yujie; Majda, Marcin; Long, Jeffrey R.; Chang, Christopher J.Science (Washington, DC, United States) (2012), 335 (6069), 698-702CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)Inorg. solids are an important class of catalysts that often derive their activity from sparse active sites that are structurally distinct from the inactive bulk. Rationally optimizing activity is therefore beholden to the challenges in studying these active sites in mol. detail. Here, we report a mol. that mimics the structure of the proposed triangular active edge site fragments of molybdenum disulfide (MoS2), a widely used industrial catalyst that has shown promise as a low-cost alternative to platinum for electrocatalytic hydrogen prodn. By leveraging the robust coordination environment of a pentapyridyl ligand, we synthesized and structurally characterized a well-defined MoIV-disulfide complex that, upon electrochem. redn., can catalytically generate hydrogen from acidic org. media as well as from acidic water.
- 17Splendiani, A.; Sun, L.; Zhang, Y. B.; Li, T. S.; Kim, J.; Chim, C. Y.; Galli, G.; Wang, F. Emerging Photoluminescence in Monolayer MoS2 Nano Lett. 2010, 10, 1271– 127517https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXjt1Sqsbs%253D&md5=7df269b35ce26d97dd8fbec1d8b6117dEmerging Photoluminescence in Monolayer MoS2Splendiani, Andrea; Sun, Liang; Zhang, Yuanbo; Li, Tianshu; Kim, Jonghwan; Chim, Chi-Yung; Galli, Giulia; Wang, FengNano Letters (2010), 10 (4), 1271-1275CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)Novel phys. phenomena can emerge in low-dimensional nanomaterials. Bulk MoS2, a prototypical metal dichalcogenide, is an indirect bandgap semiconductor with negligible photoluminescence. When the MoS2 crystal is thinned to monolayer, however, a strong photoluminescence emerges, indicating an indirect to direct bandgap transition in this d-electron system. Quantum confinement in layered d-electron materials like MoS2 provides new opportunities for engineering the electronic structure of matter at the nanoscale.
- 18Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Atomically Thin MoS2: A New Direct-gap Semiconductor Phys. Rev. Lett. 2010, 105, 13680518https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXht1Chs7zL&md5=f29a2e9692fc341d1b921f7862cf4c2aAtomically Thin MoS2. A New Direct-Gap SemiconductorMak, Kin Fai; Lee, Changgu; Hone, James; Shan, Jie; Heinz, Tony F.Physical Review Letters (2010), 105 (13), 136805/1-136805/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)The electronic properties of ultrathin crystals of MoS2 consisting of N = 1, 2,...,6 S-Mo-S monolayers were investigated by optical spectroscopy. Through characterization by absorption, photoluminescence, and photocond. spectroscopy, we trace the effect of quantum confinement on the material's electronic structure. With decreasing thickness, the indirect band gap, which lies below the direct gap in the bulk material, shifts upwards in energy by >0.6 eV. This leads to a crossover to a direct-gap material in the limit of the single monolayer. Unlike the bulk material, the MoS2 monolayer emits light strongly. The freestanding monolayer exhibits an increase in luminescence quantum efficiency by more than a factor of 104 compared with the bulk material.
- 19Yin, Z. Y.; Li, H.; Li, H.; Jiang, L.; Shi, Y. M.; Sun, Y. H.; Lu, G.; Zhang, Q.; Chen, X. D.; Zhang, H. Single-layer MoS2 Phototransistors ACS Nano 2012, 6, 74– 8019https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXhs1SmurvE&md5=28f349c4ed94a33568324f610b2dd126Single-Layer MoS2 PhototransistorsYin, Zongyou; Li, Hai; Li, Hong; Jiang, Lin; Shi, Yumeng; Sun, Yinghui; Lu, Gang; Zhang, Qing; Chen, Xiaodong; Zhang, HuaACS Nano (2012), 6 (1), 74-80CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)A new phototransistor based on the mech. exfoliated single-layer MoS2 nanosheet is fabricated, and its light-induced elec. properties were studied. Photocurrent generated from the phototransistor is solely detd. by the illuminated optical power at a const. drain or gate voltage. The switching behavior of photocurrent generation and annihilation can be completely finished within ∼50 ms, and it shows good stability. Esp., the single-layer MoS2 phototransistor exhibits a better photoresponsivity as compared with the graphene-based device. The unique characteristics of incident-light control, prompt photoswitching, and good photoresponsivity from the MoS2 phototransistor pave an avenue to develop the single-layer semiconducting materials for multifunctional optoelectronic device applications in the future.
- 20Choi, W.; Cho, M. Y.; Konar, A.; Lee, J. H.; Cha, G. B.; Hong, S. C.; Kim, S.; Kim, J.; Jena, D.; Joo, J.et al. High-detectivity Multilayer MoS2 Phototransistors with Spectral Response from Ultraviolet to Infrared Adv. Mater. 2012, 24, 5832– 583620https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38Xht1aqt73E&md5=e1d2c1ec62f3f02c6c6c83fa25851ceaHigh-detectivity multilayer MoS2 phototransistors with spectral response from ultraviolet to infraredChoi, Woong; Cho, Mi Yeon; Konar, Aniruddha; Lee, Jong Hak; Cha, Gi-Beom; Hong, Soon Cheol; Kim, Sangsig; Kim, Jeongyong; Jena, Debdeep; Joo, Jinsoo; Kim, SunkookAdvanced Materials (Weinheim, Germany) (2012), 24 (43), 5832-5836CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)This study relatest to the optoelectronic properties of multilayer MoS2 TFTs and shows a compelling case of multilayer MoS2 phototransistors for applications in photodetectors. In particular, the interesting optoelectronic properties of our multilayer MoS2 phototransistors could potentially lead to their integration into touch screen panels for flat panel or flexible display devices.
- 21Lopez-Sanchez, O.; Lembke, D.; Kayci, M.; Radenovic, A.; Kis, A. Ultrasensitive Photodetectors Based on Monolayer MoS2 Nat. Nanotechnol. 2013, 8, 497– 50121https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXptVanurY%253D&md5=10032f352964457aa0fc6b8ee9d4a486Ultrasensitive photodetectors based on monolayer MoS2Lopez-Sanchez, Oriol; Lembke, Dominik; Kayci, Metin; Radenovic, Aleksandra; Kis, AndrasNature Nanotechnology (2013), 8 (7), 497-501CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)Ultrasensitive monolayer MoS2 phototransistors with improved device mobility and ON current are demonstrated. The devices show a max. external photoresponsivity of 880 A W-1 at λ = 561 nm and a photoresponse at 400-680 nm. With recent developments in large-scale prodn. techniques such as liq.-scale exfoliation and CVD-like growth, MoS2 shows important potential for applications in MoS2-based integrated optoelectronic circuits, light sensing, biomedical imaging, video recording and spectroscopy.
- 22Mak, K. F.; He, K. L.; Lee, C.; Lee, G. H.; Hone, J.; Heinz, T. F.; Shan, J. Tightly Bound Trions in Monolayer MoS2 Nat. Mater. 2013, 12, 207– 21122https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhslGku7jJ&md5=df9e334599ae4b69e243ff181e894daaTightly bound trions in monolayer MoS2Mak, Kin Fai; He, Keliang; Lee, Changgu; Lee, Gwan Hyoung; Hone, James; Heinz, Tony F.; Shan, JieNature Materials (2013), 12 (3), 207-211CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)Two-dimensional (2D) at. crystals, such as graphene and transition-metal dichalcogenides, have emerged as a new class of materials with remarkable phys. properties. In contrast to graphene, monolayer MoS2 is a noncentrosym. material with a direct energy gap. Strong photoluminescence, a current on/off ratio exceeding 108 in field-effect transistors, and efficient valley and spin control by optical helicity have recently been demonstrated in this material. Here the authors report the spectroscopic identification in a monolayer MoS2 field-effect transistor of tightly bound neg. trions, a quasiparticle composed of 2 electrons and a hole. These quasiparticles, which can be optically created with valley and spin polarized holes, have no analog in conventional semiconductors. They also possess a large binding energy (∼ 20 meV), rendering them significant even at room temp. Results open up possibilities both for fundamental studies of many-body interactions and for optoelectronic and valleytronic applications in 2-dimensional at. crystals.
- 23Coleman, J. N.; Lotya, M.; O’Neill, A.; Bergin, S. D.; King, P. J.; Khan, U.; Young, K.; Gaucher, A.; De, S.; Smith, R. J.et al. Two-dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials Science 2011, 331, 568– 57123https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXhtlWisLY%253D&md5=7bd4a9da1b4f81f2caa3d1159dd8a5c7Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered MaterialsColeman, Jonathan N.; Lotya, Mustafa; O'Neill, Arlene; Bergin, Shane D.; King, Paul J.; Khan, Umar; Young, Karen; Gaucher, Alexandre; De, Sukanta; Smith, Ronan J.; Shvets, Igor V.; Arora, Sunil K.; Stanton, George; Kim, Hye-Young; Lee, Kangho; Kim, Gyu Tae; Duesberg, Georg S.; Hallam, Toby; Boland, John J.; Wang, Jing Jing; Donegan, John F.; Grunlan, Jaime C.; Moriarty, Gregory; Shmeliov, Aleksey; Nicholls, Rebecca J.; Perkins, James M.; Grieveson, Eleanor M.; Theuwissen, Koenraad; McComb, David W.; Nellist, Peter D.; Nicolosi, ValeriaScience (Washington, DC, United States) (2011), 331 (6017), 568-571CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)If they could be easily exfoliated, layered materials would become a diverse source of two-dimensional crystals whose properties would be useful in applications ranging from electronics to energy storage. Layered compds. such as MoS2, WS2, MoSe2, MoTe2, TaSe2, NbSe2, NiTe2, BN, and Bi2Te3 can be efficiently dispersed in common solvents and can be deposited as individual flakes or formed into films. Electron microscopy strongly suggests that the material is exfoliated into individual layers. By blending this material with suspensions of other nanomaterials or polymer solns., the authors can prep. hybrid dispersions or composites, which can be cast into films. WS2 and MoS2 effectively reinforce polymers, whereas WS2/carbon nanotube hybrid films have high cond., leading to promising thermoelec. properties.
- 24Wang, H.; Yu, L. L.; Lee, Y. H.; Shi, Y. M.; Hsu, A.; Chin, M. L.; Li, L. J.; Dubey, M.; Kong, J.; Palacios, T. Integrated Circuits Based on Bilayer MoS2 Transistors Nano Lett. 2012, 12, 4674– 468024https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhtFCht7jL&md5=36edd940f16c94fe388951be66b46e40Integrated Circuits Based on Bilayer MoS2 TransistorsWang, Han; Yu, Lili; Lee, Yi-Hsien; Shi, Yumeng; Hsu, Allen; Chin, Matthew L.; Li, Lain-Jong; Dubey, Madan; Kong, Jing; Palacios, TomasNano Letters (2012), 12 (9), 4674-4680CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)Two-dimensional (2D) materials, such as molybdenum disulfide (MoS2), have been shown to exhibit excellent elec. and optical properties. The semiconducting nature of MoS2 allows it to overcome the shortcomings of zero-bandgap graphene, while still sharing many of graphene's advantages for electronic and optoelectronic applications. Discrete electronic and optoelectronic components, such as field-effect transistors, sensors, and photodetectors made from few-layer MoS2 show promising performance as potential substitute of Si in conventional electronics and of org. and amorphous Si semiconductors in ubiquitous systems and display applications. An important next step is the fabrication of fully integrated multistage circuits and logic building blocks on MoS2 to demonstrate its capability for complex digital logic and high-frequency ac applications. This paper demonstrates an inverter, a NAND gate, a static random access memory, and a five-stage ring oscillator based on a direct-coupled transistor logic technol. The circuits comprise between 2 to 12 transistors seamlessly integrated side-by-side on a single sheet of bilayer MoS2. Both enhancement-mode and depletion-mode transistors were fabricated thanks to the use of gate metals with different work functions.
- 25Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-layer MoS2 Transistors Nat. Nanotechnol. 2011, 6, 147– 15025https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXislCjsro%253D&md5=555366539a8a87d074a69674aafaf315Single-layer MoS2 transistorsRadisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A.Nature Nanotechnology (2011), 6 (3), 147-150CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)Two-dimensional materials are attractive for use in next-generation nanoelectronic devices because, compared to 1D materials, it is relatively easy to fabricate complex structures from them. The most widely studied 2D material is graphene, both because of its rich physics and its high mobility. However, pristine graphene does not have a bandgap, a property that is essential for many applications, including transistors. Engineering a graphene bandgap increases fabrication complexity and either reduces mobilities to the level of strained Si films or requires high voltages. Although single layers of MoS2 have a large intrinsic bandgap of 1.8 eV, previously reported mobilities in the 0.5-3 cm2 V-1 s-1 range are too low for practical devices. Here, we use a HfO2 gate dielec. to demonstrate a room-temp. single-layer MoS2 mobility of at least 200 cm2 V-1 s-1, similar to that of graphene nanoribbons, and demonstrate transistors with room-temp. current on/off ratios of 1 × 108 and ultralow standby power dissipation. Because monolayer MoS2 has a direct bandgap, it can be used to construct interband tunnel FETs, which offer lower power consumption than classical transistors. Monolayer MoS2 could also complement graphene in applications that require thin transparent semiconductors, such as optoelectronics and energy harvesting.
- 26Alam, K.; Lake, R. K. Monolayer MoS2 Transistors beyond the Technology Road Map IEEE Trans. Electron Devices 2012, 59, 3250– 3254There is no corresponding record for this reference.
- 27Radisavljevic, B.; Whitwick, M. B.; Kis, A. Small-signal Amplifier Based On Single-layer MoS2 Appl. Phys. Lett. 2012, 101, 04310327https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhtVOgur7L&md5=a92ebe5c74d0b5e26a12d76e9324576dSmall-signal amplifier based on single-layer MoS2Radisavljevic, Branimir; Whitwick, Michael B.; Kis, AndrasApplied Physics Letters (2012), 101 (4), 043103/1-043103/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)We demonstrate the operation of an analog small-signal amplifier based on single-layer MoS2, a semiconducting analog of graphene. Our device consists of 2 transistors integrated on the same piece of single-layer MoS2. The high intrinsic band gap of 1.8 eV allows MoS2-based amplifiers to operate with a room temp. gain of 4. The amplifier operation is demonstrated for the frequencies of input signal up to 2 kHz preserving the gain higher than 1. MoS2 can effectively amplify signals and that it could be used for advanced analog circuits based on 2D materials. (c) 2012 American Institute of Physics.
- 28Radisavljevic, B.; Kis, A. Mobility Engineering and a Metal-insulator Transition in Monolayer MoS2 Nat. Mater. 2013, 12, 815– 82028https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXpvVyqsbg%253D&md5=4e7fd06bc5f8097ac80f5978694581abMobility engineering and a metal-insulator transition in monolayer MoS2Radisavljevic, Branimir; Kis, AndrasNature Materials (2013), 12 (9), 815-820CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)Two-dimensional (2D) materials are a new class of materials with interesting phys. properties and applications ranging from nanoelectronics to sensing and photonics. In addn. to graphene, the most studied 2-dimensional material, monolayers of other layered materials such as semiconducting dichalcogenides MoS2 or WSe2 are gaining in importance as promising channel materials for field-effect transistors (FETs). The presence of a direct bandgap in monolayer MoS2 due to quantum-mech. confinement allows room-temp. FETs with an on/off ratio exceeding 108. The presence of high-κ dielecs. in these devices enhanced their mobility, but the mechanisms are not well understood. Here, the authors report on elec. transport measurements on MoS2 FETs in different dielec. configurations. The dependence of mobility on temp. shows clear evidence of the strong suppression of charged-impurity scattering in dual-gate devices with a top-gate dielec. At the same time, phonon scattering shows a weaker than expected temp. dependence. High levels of doping achieved in dual-gate devices also allow the observation of a metal-insulator transition in monolayer MoS2 due to strong electron-electron interactions. The authors' work opens up the way to further improvements in 2-dimensional semiconductor performance and introduces MoS2 as an interesting system for studying correlation effects in mesoscopic systems.
- 29Zhang, W. J.; Chuu, C. P.; Huang, J. K.; Chen, C. H.; Tsai, M. L.; Chang, Y. H.; Liang, C. T.; Chen, Y. Z.; Chueh, Y. L.; He, J. H.et al. Ultrahigh-gain Photodetectors Based on Atomically Thin Graphene-MoS2 Heterostructures Sci. Rep. 2014, 4, 382629https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXjslCis7c%253D&md5=721ff0731857e8c68d85a5ae137fcb31Ultrahigh-Gain Photodetectors Based on Atomically Thin Graphene-MoS2 HeterostructuresZhang, Wenjing; Chuu, Chih-Piao; Huang, Jing-Kai; Chen, Chang-Hsiao; Tsai, Meng-Lin; Chang, Yung-Huang; Liang, Chi-Te; Chen, Yu-Ze; Chueh, Yu-Lun; He, Jr-Hau; Chou, Mei-Yin; Li, Lain-JongScientific Reports (2014), 4 (), 3826/1-3826/8CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)Due to its high carrier mobility, broadband absorption, and fast response time, the semi-metallic graphene is attractive for optoelectronics. Another two-dimensional semiconducting material molybdenum disulfide (MoS2) is also known as light- sensitive. Here we show that a large-area and continuous MoS2 monolayer is achievable using a CVD method and graphene is transferable onto MoS2. We demonstrate that a photodetector based on the graphene/MoS2 heterostructure is able to provide a high photogain greater than 108. Our expts. show that the electron-hole pairs are produced in the MoS2 layer after light absorption and subsequently sepd. across the layers. Contradictory to the expectation based on the conventional built-in elec. field model for metal-semiconductor contacts, photoelectrons are injected into the graphene layer rather than trapped in MoS2 due to the presence of a perpendicular effective elec. field caused by the combination of the built-in elec. field, the applied electrostatic field, and charged impurities or adsorbates, resulting in a tuneable photoresponsivity.
- 30Bertolazzi, S.; Krasnozhon, D.; Kis, A.; Nonvolatile Memory Cells Based On MoS2/Graphene Heterostructures ACS Nano 2013, 7, 3246– 3252There is no corresponding record for this reference.
- 31Kaasbjerg, K.; Thygesen, K. S.; Jacobsen, K. W. Phonon-limited Mobility In N-type Single-layer MoS2 from First Principles Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 11531731https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38Xpt1ynu7c%253D&md5=61d96a0b9479a4c9c5241b42609fcdd3Phonon-limited mobility in n-type single-layer MoS2 from first principlesKaasbjerg, Kristen; Thygesen, Kristian S.; Jacobsen, Karsten W.Physical Review B: Condensed Matter and Materials Physics (2012), 85 (11), 115317/1-115317/16CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)We study the phonon-limited mobility in intrinsic n-type single-layer MoS2 for temps. T > 100 K. The materials properties including the electron-phonon interaction are calcd. from first principles and the deformation potentials and Frohlich interaction in single-layer MoS2 are established. The calcd. room-temp. mobility of ∼410 cm2V-1s-1 is found to be dominated by optical phonon scattering via intra and intervalley deformation potential couplings and the Frohlich interaction. The mobility is weakly dependent on the carrier d. and follows a μ ∼ T-γ temp. dependence with γ = 1.69 at room temp. It is shown that a quenching of the characteristic homopolar mode, which is likely to occur in top-gated samples, increases the mobility with ∼70 cm2V-1s-1 and can be obsd. as a decrease in the exponent to γ = 1.52. In comparison to recent exptl. findings for the mobility in single-layer MoS2 (∼200 cm2V-1s-1), our results indicate that mobilities close to the intrinsic phonon-limited mobility can be achieved in two-dimensional materials via dielec. engineering that effectively screens static Coulomb scattering on, e.g., charged impurities.
- 32Das, S.; Chen, H. Y.; Penumatcha, A. V.; Appenzeller, J. High Performance Multilayer MoS2 Transistors with Scandium Contacts Nano Lett. 2013, 13, 100– 10532https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhvVersLzO&md5=e5e71a0be2d5d45f7cc4ff9149b3cbb4High Performance Multilayer MoS2 Transistors with Scandium ContactsDas, Saptarshi; Chen, Hong-Yan; Penumatcha, Ashish Verma; Appenzeller, JoergNano Letters (2013), 13 (1), 100-105CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)While there was growing interest in two-dimensional (2-D) crystals other than graphene, evaluating their potential usefulness for electronic applications is still in its infancy due to the lack of a complete picture of their performance potential. The focus of this article is on contacts. Through a proper understanding and design of source/drain contacts and the right choice of no. of MoS2 layers the excellent intrinsic properties of this 2-dimensional material can be harvested. Using scandium contacts on 10-nm-thick exfoliated MoS2 flakes that are covered by a 15. nm Al2O3 film, high effective mobilities of 700 cm2/(V s) are achieved at room temp. This breakthrough is largely attributed to the fact that the authors succeeded in eliminating contact resistance effects that limited the device performance in the past unrecognized. In fact, the apparent linear dependence of current on drain voltage had mislead researchers to believe that a truly ohmic contact had already been achieved, a misconception that the authors also elucidate.
- 33Shi, H.; Pan, H.; Zhang, Y. W.; Yakobson, B. I. Quasiparticle Band Structures and Optical Properties of Strained Monolayer MoS2 and WS2 Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87, 15530433https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXovVens78%253D&md5=2bdbe3b5eb1a87afbe3d90cb73ca70ecQuasiparticle band structures and optical properties of strained monolayer MoS2 and WS2Shi, Hongliang; Pan, Hui; Zhang, Yong-Wei; Yakobson, Boris I.Physical Review B: Condensed Matter and Materials Physics (2013), 87 (15), 155304/1-155304/8CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)The quasiparticle (QP) band structures of both strainless and strained monolayer MoS2 are investigated using more accurate many-body perturbation GW theory and maximally localized Wannier functions (MLWFs) approach. By solving the Bethe-Salpeter equation (BSE) including excitonic effects on top of the partially self-consistent GW0 (scGW0) calcn., the predicted optical gap magnitude is in good agreement with available exptl. data. With increasing strain, the exciton binding energy is nearly unchanged, while optical gap is reduced significantly. The scGW0 and BSE calcns. are also performed on monolayer WS2, similar characteristics are predicted and WS2 possesses the lightest effective mass at the same strain among monolayers Mo(S,Se) and W(S,Se). Our results also show that the electron effective mass decreases as the tensile strain increases, resulting in an enhanced carrier mobility. The present calcn. results suggest a viable route to tune the electronic properties of monolayer transition-metal dichalcogenides (TMDs) using strain engineering for potential applications in high performance electronic devices.
- 34Allain, A.; Kis, A. Electron and Hole Mobilities in Single-layer WSe2 ACS Nano 2014, 8, 7180– 718534https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhtVahtrrF&md5=ff021e02a26db6df8981fc27101b2e3cElectron and Hole Mobilities in Single-Layer WSe2Allain, Adrien; Kis, AndrasACS Nano (2014), 8 (7), 7180-7185CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)Single-layer transition metal dichalcogenide WSe2 has recently attracted a lot of attention because it is a 2-dimensional semiconductor with a direct band gap. Due to low doping levels, it is intrinsic and shows ambipolar transport. This opens up the possibility to realize devices with the Fermi level located in the valence band, where the spin/valley coupling is strong and leads to new and interesting physics. As a consequence of its intrinsically low doping, large Schottky barriers form between WSe2 and metal contacts, which impede the injection of charges at low temps. Here, the authors report on the study of single-layer WSe2 transistors with a polymer electrolyte gate (PEO: LiClO4). Polymer electrolytes allow the charge carrier densities to be modulated to very high values, allowing the observation of both the electron- and the hole-doped regimes. Also, the authors' ohmic contacts formed at low temps. allow the authors to study the temp. dependence of electron and hole mobilities. At high electron densities, a reentrant insulating regime is also obsd., a feature which is absent at high hole densities.
- 35Huang, J. K.; Pu, J.; Hsu, C. L.; Chiu, M. H.; Juang, Z. Y.; Chang, Y. H.; Chang, W. H.; Iwasa, Y.; Takenobu, T.; Li, L. J. Large-area Synthesis of Highly Crystalline WSe2 Mono Layers and Device Applications ACS Nano 2014, 8, 923– 930There is no corresponding record for this reference.
- 36Jones, A. M.; Yu, H. Y.; Ghimire, N. J.; Wu, S. F.; Aivazian, G.; Ross, J. S.; Zhao, B.; Yan, J. Q.; Mandrus, D. G.; Xiao, D.et al. Optical Generation of Excitonic Valley Coherence in Monolayer WSe2 Nat. Nanotechnol. 2013, 8, 63436https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXht1CgsLjE&md5=d6f50e4ea06c115268e08c4e324b35d7Optical generation of excitonic valley coherence in monolayer WSe2Jones, Aaron M.; Yu, Hongyi; Ghimire, Nirmal J.; Wu, Sanfeng; Aivazian, Grant; Ross, Jason S.; Zhao, Bo; Yan, Jiaqiang; Mandrus, David G.; Xiao, Di; Yao, Wang; Xu, XiaodongNature Nanotechnology (2013), 8 (9), 634-638CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)As a consequence of degeneracies arising from crystal symmetries, it is possible for electron states at band-edges ( valleys') to have addnl. spin-like quantum nos. An important question is whether coherent manipulation can be performed on such valley pseudospins, analogous to that implemented using true spin, in the quest for quantum technologies. Here, we show that valley coherence can be generated and detected. Because excitons in a single valley emit circularly polarized photons, linear polarization can only be generated through recombination of an exciton in a coherent superposition of the two valley states. Using monolayer semiconductor WSe2 devices, we first establish the circularly polarized optical selection rules for addressing individual valley excitons and trions. We then demonstrate coherence between valley excitons through the observation of linearly polarized luminescence, whose orientation coincides with that of the linearly polarized excitation, for any given polarization angle. In contrast, the corresponding photoluminescence from trions is not obsd. to be linearly polarized, consistent with the expectation that the emitted photon polarization is entangled with valley pseudospin. The ability to address coherence, in addn. to valley polarization, is a step forward towards achieving quantum manipulation of the valley index necessary for coherent valleytronics.
- 37Yoshida, S.; Terada, Y.; Yokota, M.; Takeuchi, O.; Meray, Y.; Shigekawa, H. Direct Probing of Transient Photocurrent Dynamics in P-WSe2 By Time-resolved Scanning Tunneling Microscopy Appl. Phys. Express 2013, 6, 016601There is no corresponding record for this reference.
- 38Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect In Atomically Thin Carbon Films Science 2004, 306, 666– 66938https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2cXos1Kqt70%253D&md5=488da13500bf24e8fc419052dc1a9e84Electric Field Effect in Atomically Thin Carbon FilmsNovoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A.Science (Washington, DC, United States) (2004), 306 (5696), 666-669CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)The authors describe monocryst. graphitic films, which are a few atoms thick but are nonetheless stable under ambient conditions, metallic, and of remarkably high quality. The films are a two-dimensional semimetal with a tiny overlap between valence and conductance bands, and they exhibit a strong ambipolar elec. field effect such that electrons and holes in concns. up to 1013 per square centimeter and with room-temp. mobilities of ∼10,000 square centimeters per V-second can be induced by applying gate voltage.
- 39Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. Two-dimensional Gas of Massless Dirac Fermions in Graphene Nature 2005, 438, 197– 20039https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2MXhtF2nsrnI&md5=56138229370ff26ece1857a049f00f53Two-dimensional gas of massless Dirac fermions in grapheneNovoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A.Nature (London, United Kingdom) (2005), 438 (7065), 197-200CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)Quantum electrodynamics (resulting from the merger of quantum mechanics and relativity theory) has provided a clear understanding of phenomena ranging from particle physics to cosmol. and from astrophysics to quantum chem. The ideas underlying quantum electrodynamics also influence the theory of condensed matter, but quantum relativistic effects are usually minute in the known exptl. systems that can be described accurately by the non-relativistic Schroedinger equation. Here we report an exptl. study of a condensed-matter system (graphene, a single at. layer of carbon) in which electron transport is essentially governed by Dirac's (relativistic) equation. The charge carriers in graphene mimic relativistic particles with zero rest mass and have an effective 'speed of light' c* ≈ 106 m s-1. Our study reveals a variety of unusual phenomena that are characteristic of two-dimensional Dirac fermions. In particular we have obsd. the following: first, graphene's cond. never falls below a min. value corresponding to the quantum unit of conductance, even when concns. of charge carriers tend to zero; second, the integer quantum Hall effect in graphene is anomalous in that it occurs at half-integer filling factors; and third, the cyclotron mass mc of massless carriers in graphene is described by E = mcc*2. This two-dimensional system is not only interesting in itself but also allows access to the subtle and rich physics of quantum electrodynamics in a bench-top expt.
- 40Tonndorf, P.; Schmidt, R.; Bottger, P.; Zhang, X.; Borner, J.; Liebig, A.; Albrecht, M.; Kloc, C.; Gordan, O.; Zahn, D. R. T.et al. Photoluminescence Emission and Raman Response of Monolayer MoS2, MoSe2, And WSe2 Opt. Express 2013, 21, 4908– 491640https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXjslyrurY%253D&md5=d17f7bdbba58f863220eefdeb116e5caPhotoluminescence emission and Raman response of monolayer MoS2, MoSe2, and WSe2Tonndorf, Philipp; Schmidt, Robert; Boettger, Philipp; Zhang, Xiao; Boerner, Janna; Liebig, Andreas; Albrecht, Manfred; Kloc, Christian; Gordan, Ovidiu; Zahn, Dietrich R. T.; de Vasconcellos, Steffen Michaelis; Bratschitsch, RudolfOptics Express (2013), 21 (4), 4908-4916CODEN: OPEXFF; ISSN:1094-4087. (Optical Society of America)We mech. exfoliate mono- and few-layers of the transition metal dichalcogenides molybdenum disulfide, molybdenum diselenide, and tungsten diselenide. The exact no. of layers is unambiguously detd. by at. force microscopy and high-resoln. Raman spectroscopy. Strong photoluminescence emission is caused by the transition from an indirect band gap semiconductor of bulk material to a direct band gap semiconductor in atomically thin form.
- 41Zhao, W. J.; Ghorannevis, Z.; Chu, L. Q.; Toh, M. L.; Kloc, C.; Tan, P. H.; Eda, G. Evolution of Electronic Structure in Atomically Thin Sheets of WS2 and WSe2 ACS Nano 2013, 7, 791– 79741https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhvVGrur%252FM&md5=1077846a4fad1eb6029ced7f87b1a708Evolution of Electronic Structure in Atomically Thin Sheets of WS2 and WSe2Zhao, Weijie; Ghorannevis, Zohreh; Chu, Leiqiang; Toh, Minglin; Kloc, Christian; Tan, Ping-Heng; Eda, GokiACS Nano (2013), 7 (1), 791-797CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)Geometrical confinement effect in exfoliated sheets of layered materials leads to significant evolution of energy dispersion in mono- to few-layer thickness regime. Molybdenum disulfide (MoS2) was recently found to exhibit indirect-to-direct gap transition when the thickness is reduced to a single monolayer. Emerging photoluminescence (PL) from monolayer MoS2 opens up opportunities for a range of novel optoelectronic applications of the material. Here we report differential reflectance and PL spectra of mono- to few-layer WS2 and WSe2 that indicate that the band structure of these materials undergoes similar indirect-to-direct gap transition when thinned to a single monolayer. The transition is evidenced by distinctly enhanced PL peak centered at 630 and 750 nm in monolayer WS2 and WSe2, resp. Few-layer flakes are found to exhibit comparatively strong indirect gap emission along with direct gap hot electron emission, suggesting high quality of synthetic crystals prepd. by a chem. vapor transport method. Fine absorption and emission features and their thickness dependence suggest a strong effect of Se p-orbitals on the d electron band structure as well as interlayer coupling in WSe2.
- 42Lee, Y. H.; Zhang, X. Q.; Zhang, W. J.; Chang, M. T.; Lin, C. T.; Chang, K. D.; Yu, Y. C.; Wang, J. T. W.; Chang, C. S.; Li, L. J.et al. Synthesis of Large-area MoS2 Atomic Layers with Chemical Vapor Deposition Adv. Mater. 2012, 24, 2320– 232542https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XkvVKiur8%253D&md5=7b5ca7016ced6baa546a93be5bbe8589Synthesis of Large-Area MoS2 Atomic Layers with Chemical Vapor DepositionLee, Yi-Hsien; Zhang, Xin-Quan; Zhang, Wenjing; Chang, Mu-Tung; Lin, Cheng-Te; Chang, Kai-Di; Yu, Ya-Chu; Wang, Jacob Tse-Wei; Chang, Chia-Seng; Li, Lain-Jong; Lin, Tsung-WuAdvanced Materials (Weinheim, Germany) (2012), 24 (17), 2320-2325CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)Large-area MoS2 at. layers are synthesized on SiO2 substrates by chem. vapor deposition using MoO3 and S powders as the reactants. Optical, microscopic and elec. measurements suggest that the synthetic process leads to the growth of MoS2 monolayer. The TEM images verify that the synthesized MoS2 sheets are highly cryst. To check for elec. performance bottom-gated transistors on silica/silicon using photolithog. was fabricated directly on top of the MoS2 sheets. The transfer curve (drain current vs. gate voltage) was computed and field effect mobility was detd. from anal. of the curve.
- 43Lee, Y. H.; Yu, L.; Wang, H.; Fang, W.; I Ling, X.; Shi, Y.; Lin, C. T.; Huang, J. K.; Chang, M. T.; Chang, C. S.et al. Synthesis and Transfer of Single-layer Transition Metal Disulfides on Diverse Surfaces Nano Lett. 2013, 13, 185243https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXktF2gtr8%253D&md5=c3ef496ee3dd9ec34ec96b8f1be19af2Synthesis and Transfer of Single-Layer Transition Metal Disulfides on Diverse SurfacesLee, Yi-Hsien; Yu, Lili; Wang, Han; Fang, Wenjing; Ling, Xi; Shi, Yumeng; Lin, Cheng-Te; Huang, Jing-Kai; Chang, Mu-Tung; Chang, Chia-Seng; Dresselhaus, Mildred; Palacios, Tomas; Li, Lain-Jong; Kong, JingNano Letters (2013), 13 (4), 1852-1857CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)High-quality MS2 (M = Mo, W) monolayers have been prepd. using ambient-pressure chem. vapor deposition (APCVD) with the seeding of perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS). The growth of a MS2 monolayer is achieved on various surfaces with a significant flexibility to surface corrugation. Electronic transport and optical performances of the as-grown MS2 monolayers are comparable to those of exfoliated MS2 monolayers. A robust technique for transferring the MS2 monolayer samples to diverse surfaces was developed which may stimulate the progress on the class of materials and open a new route toward the synthesis of various novel hybrid structures with LTMD monolayer and functional materials.
- 44Liu, K. K.; Zhang, W. J.; Lee, Y. H.; Lin, Y. C.; Chang, M. T.; Su, C.; Chang, C. S.; Li, H.; Shi, Y. M.; Zhang, H.et al. Growth of Large-area And Highly Crystalline MoS2 Thin Layers on Insulating Substrates Nano Lett. 2012, 12, 1538– 154444https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XivVOgsbg%253D&md5=d9954476e4e5a84d99e2dd4a4290b3edGrowth of Large-Area and Highly Crystalline MoS2 Thin Layers on Insulating SubstratesLiu, Keng-Ku; Zhang, Wenjing; Lee, Yi-Hsien; Lin, Yu-Chuan; Chang, Mu-Tung; Su, Ching-Yuan; Chang, Chia-Seng; Li, Hai; Shi, Yumeng; Zhang, Hua; Lai, Chao-Sung; Li, Lain-JongNano Letters (2012), 12 (3), 1538-1544CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)The two-dimensional layer of molybdenum disulfide (MoS2) has recently attracted much interest due to its direct-gap property and potential applications in optoelectronics and energy harvesting. However, the synthetic approach to obtain high-quality and large-area MoS2 at. thin layers is still rare. Here we report that the high-temp. annealing of a thermally decompd. ammonium thiomolybdate layer in the presence of sulfur can produce large-area MoS2 thin layers with superior elec. performance on insulating substrates. Spectroscopic and microscopic results reveal that the synthesized MoS2 sheets are highly cryst. The electron mobility of the bottom-gate transistor devices made of the synthesized MoS2 layer is comparable with those of the micromechanically exfoliated thin sheets from MoS2 crystals. This synthetic approach is simple, scalable, and applicable to other transition metal dichalcogenides. Meanwhile, the obtained MoS2 films are transferable to arbitrary substrates, providing great opportunities to make layered composites by stacking various atomically thin layers.
- 45Li, X. S.; Cai, W. W.; An, J. H.; Kim, S.; Nah, J.; Yang, D. X.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.et al. Large-area Synthesis of High-quality and Uniform Graphene Films on Copper Foils Science 2009, 324, 1312– 131445https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXms12gtbY%253D&md5=d5d5a8564d2dac69173cf0696d21eb3eLarge-Area Synthesis of High-Quality and Uniform Graphene Films on Copper FoilsLi, Xuesong; Cai, Weiwei; An, Jinho; Kim, Seyoung; Nah, Junghyo; Yang, Dongxing; Piner, Richard; Velamakanni, Aruna; Jung, Inhwa; Tutuc, Emanuel; Banerjee, Sanjay K.; Colombo, Luigi; Ruoff, Rodney S.Science (Washington, DC, United States) (2009), 324 (5932), 1312-1314CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)Graphene was attracting great interest because of its distinctive band structure and phys. properties. Today, graphene is limited to small sizes because it is produced mostly by exfoliating graphite. The authors grew large-area graphene films of the order of centimeters on Cu substrates by CVD using methane. The films are predominantly single-layer graphene, with a small percentage (<5%) of the area having few layers, and are continuous across Cu surface steps and grain boundaries. The low soly. of C in Cu appears to help make this growth process self-limiting. The authors also developed graphene film transfer processes to arbitrary substrates, and dual-gated field-effect transistors fabricated on Si/SiO2 substrates showed electron mobilities ≤4050 cm2/V-s at room temp.
- 46Korn, T.; Heydrich, S.; Hirmer, M.; Schmutzler, J.; Schuller, C. Low-temperature Photocarrier Dynamics in Monolayer MoS2 Appl. Phys. Lett. 2011, 99, 10210946https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXhtFCqtbvF&md5=dd2b95fed91ba88efdee15c6c5fe227aLow-temperature photocarrier dynamics in monolayer MoS2Korn, T.; Heydrich, S.; Hirmer, M.; Schmutzler, J.; Schueller, C.Applied Physics Letters (2011), 99 (10), 102109/1-102109/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)The band structure of MoS2 strongly depends on the no. of layers, and a transition from indirect to direct-gap semiconductor was obsd. recently for a single layer of MoS2. Single-layer MoS2 therefore becomes an efficient emitter of photoluminescence even at room temp. Here, we report on scanning Raman and on temp.-dependent, as well as time-resolved photoluminescence measurements on single-layer MoS2 flakes prepd. by exfoliation. We observe the emergence of 2 distinct photoluminescence peaks at low temps. The photocarrier recombination at low temps. occurs on the few-picosecond timescale, but with increasing temps., a biexponential photoluminescence decay with a longer-lived component is obsd. (c) 2011 American Institute of Physics.
- 47Sundaram, R.; Engel, M.; Lombardo, A.; Krupke, R.; Ferrari, A.; H Avouris, P.; Steiner, M. Electroluminescence in Single Layer MoS2 Nano Lett. 2013, 13, 1416– 142147https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXktlyksbg%253D&md5=bfe4b93685b4821572c00b3e67245fb9Electroluminescence in Single Layer MoS2Sundaram, R. S.; Engel, M.; Lombardo, A.; Krupke, R.; Ferrari, A. C.; Avouris, Ph.; Steiner, M.Nano Letters (2013), 13 (4), 1416-1421CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)The authors detect electroluminescence in single layer MoS2 FETs built on transparent glass substrates. By comparing the absorption, photoluminescence, and electroluminescence of the same MoS2 layer, they all involve the same excited state at 1.8 eV. The electroluminescence has pronounced threshold behavior and is localized at the contacts. Single layer MoS2, a direct band gap semiconductor, could be promising for novel optoelectronic devices, such as 2-dimensional light detectors and emitters.
- 48Wang, R.; Ruzicka, B. A.; Kumar, N.; Bellus, M. Z.; Chiu, H. Y.; Zhao, H. Ultrafast and Spatially Resolved Studies of Charge Carriers in Atomically Thin Molybdenum Disulfide Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86, 04540648https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhtlSltL3O&md5=da9321359a0928768a41a180d8a5828dUltrafast and spatially resolved studies of charge carriers in atomically thin molybdenum disulfideWang, Rui; Ruzicka, Brian A.; Kumar, Nardeep; Bellus, Matthew Z.; Chiu, Hsin-Ying; Zhao, HuiPhysical Review B: Condensed Matter and Materials Physics (2012), 86 (4), 045406/1-045406/5CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)Atomically thin molybdenum disulfide is emerging as a new nanomaterial with potential applications in the fields of electronic and photonics. Charge carrier dynamics plays an essential role in detg. its electronic and optical properties. We report spatially and temporally resolved pump-probe studies of charge carriers in atomically thin molybdenum disulfide samples fabricated by mech. exfoliation. Carriers are injected by interband absorption of a 390-nm pump pulse and detected by measuring differential reflection of a time-delayed and spatially scanned probe pulse that is tuned to an exciton transition. Several parameters on charge carrier dynamics are deduced, including carrier lifetime, diffusion coeff., diffusion length, and mobility.
- 49Shi, H. Y.; Yan, R. S.; Bertolazzi, S.; Brivio, J.; Gao, B.; Kis, A.; Jena, D.; Xing, H. G.; Huang, L. B. Exciton Dynamics in Suspended Mono Layer And Few-layer MoS2 2D Crystals ACS Nano 2013, 7, 1072– 1080There is no corresponding record for this reference.
- 50Lee, C.; Yan, H.; Brus, L. E.; Heinz, T. F.; Hone, J.; Ryu, S. Anomalous Lattice Vibrations of Single- and Few-layer MoS2 ACS Nano 2010, 4, 2695– 270050https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXkslKktLc%253D&md5=2bf8b27f754f40e5cdf890343171555fAnomalous Lattice Vibrations of Single- and Few-Layer MoS2Lee, Changgu; Yan, Hugen; Brus, Louis E.; Heinz, Tony F.; Hone, James; Ryu, SunminACS Nano (2010), 4 (5), 2695-2700CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)Molybdenum disulfide (MoS2) of single- and few-layer thickness was exfoliated on SiO2/Si substrate and characterized by Raman spectroscopy. The no. of S-Mo-S layers of the samples was independently detd. by contact-mode at. force microscopy. Two Raman modes, E12g and A1g, exhibited sensitive thickness dependence, with the frequency of the former decreasing and that of the latter increasing with thickness. The results provide a convenient and reliable means for detg. layer thickness with at.-level precision. The opposite direction of the frequency shifts, which cannot be explained solely by van der Waals interlayer coupling, is attributed to Coulombic interactions and possible stacking-induced changes of the intralayer bonding. This work exemplifies the evolution of structural parameters in layered materials in changing from the three-dimensional to the two-dimensional regime.
- 51Ulbricht, R.; Hendry, E.; Shan, J.; Heinz, T. F.; Bonn, M. Carrier Dynamics In Semiconductors Studied with Time-resolved Terahertz Spectroscopy Rev. Mod. Phys. 2011, 83, 543– 58651https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXos1eitrc%253D&md5=834abf847f8a88a90b38ebd7f24e872dCarrier dynamics in semiconductors studied with time-resolved terahertz spectroscopyUlbricht, Ronald; Hendry, Euan; Shan, Jie; Heinz, Tony F.; Bonn, MischaReviews of Modern Physics (2011), 83 (2), 543-586CODEN: RMPHAT; ISSN:0034-6861. (American Physical Society)A review. Time-resolved, pulsed terahertz spectroscopy has developed into a powerful tool to study charge carrier dynamics in semiconductors and semiconductor structures over the past decades. Covering the energy range from a few to about 100 meV, terahertz radiation is sensitive to the response of charge quasiparticles, e.g., free carriers, polarons, and excitons. The distinct spectral signatures of these different quasiparticles in the THz range allow their discrimination and characterization using pulsed THz radiation. This frequency region is also well suited for the study of phonon resonances and intraband transitions in low-dimensional systems. Moreover, using a pump-probe scheme, it is possible to monitor the nonequil. time evolution of carriers and low-energy excitations with sub-ps time resoln. Being an all-optical technique, terahertz time-domain spectroscopy is contact-free and noninvasive and hence suited to probe the cond. of, particularly, nanostructured materials that are difficult or impossible to access with other methods. The latest developments in the application of terahertz time-domain spectroscopy to bulk and nanostructured semiconductors are reviewed.
- 52Lloyd-Hughes, J.; Jeon, T.-I. A Review of the Terahertz Conductivity of Bulk and Nano-materials J. Infrared, Millimeter, Terahertz Waves 2012, 33, 87152https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhtFSlt7rK&md5=2275551b24f861b4f325229e2989d751A Review of the Terahertz Conductivity of Bulk and Nano-MaterialsLloyd-Hughes, James; Jeon, Tae-InJournal of Infrared, Millimeter, and Terahertz Waves (2012), 33 (9), 871-925CODEN: JIMTC4; ISSN:1866-6892. (Springer)We review pioneering and recent studies of the cond. of solid state systems at terahertz frequencies. A variety of theor. formalisms that describe the terahertz cond. of bulk, mesoscopic and nanoscale materials are outlined, and their validity and limitations are given. Exptl. highlights are discussed from studies of inorg. semiconductors, org. materials (such as graphene, carbon nanotubes and polymers), metallic films and strongly correlated electron systems including superconductors.
- 53Grabtchak, S. Y.; Cocivera, M. Contactless Microwave Study of Dispersive Transport in Thin Film CdSe J. Appl. Phys. 1996, 79, 786– 79353https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK28XltFKhsQ%253D%253D&md5=36d886a799e2adb70ea1eab52338c0dfContactless microwave study of dispersive transport in thin film CdSeGrabtchak, Serguei Yu; Cocivera, MichaelJournal of Applied Physics (1996), 79 (2), 786-93CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)The contactless microwave technique was used to measure light-induced transients in the power absorbed by thin films of polycryst. CdSe. Because the rise time of the microwave cavity was 60 ns, the anal. was limited to 100 ns or longer. Measurement of these transients at a no. of fixed frequencies across the dark resonance frequency made reconstruction of the difference signal possible. This signal, which represents the difference between the dark and light Lorentz resonance curves, was detd. at various times during the decay. Anal. of these signals provided the time dependence for the changes in the real and imaginary parts of the dielec. const., which correspond to the densities of the trapped and free electrons. The decays of these parameters were characterized by three time domains. At the shortest times, the two parameters did not have the same time dependence. At intermediate times, the densities of both the trapped and free electrons had the same time dependence characterized by a power law decay, and a mechanism consistent with these results involves rapid equilibration between the free electrons and those in the shallow traps. Decay in this region was consistent with a dispersive transport mechanism. Intensity effects indicate satn. of the shallow traps. The 3rd region occurred at the break in the power law dependence indicating a bimol. recombination process. Measurements at higher temps. indicate a change from a bimol. to a monomol. recombination mechanism.
- 54Parkinson, P.; Joyce, H. J.; Gao, Q.; Tan, H. H.; Zhang, X.; Zou, J.; Jagadish, C.; Herz, L. M.; Johnston, M. B. Carrier Lifetime and Mobility Enhancement in Nearly Defect-free Core-shell Nanowires Measured Using Time-resolved Terahertz Spectroscopy Nano Lett. 2009, 9, 3349– 335354https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXptVCks7Y%253D&md5=dfce7044b0801cc0282698055646ce6eCarrier lifetime and mobility enhancement in nearly defect-free core-shell nanowires measured using time-resolved terahertz spectroscopyParkinson, Patrick; Joyce, Hannah J.; Gao, Qiang; Tan, Hark Hoe; Zhang, Xin; Zou, Jin; Jagadish, Chennupati; Herz, Laura M.; Johnston, Michael B.Nano Letters (2009), 9 (9), 3349-3353CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)We have used transient terahertz photocond. measurements to assess the efficacy of 2-temp. growth and core-shell encapsulation techniques on the electronic properties of GaAs nanowires. 2-Temp. growth of the GaAs core leads to an almost doubling in charge-carrier mobility and a tripling of carrier lifetime. In addn., overcoating the GaAs core with a larger-bandgap material is shown to reduce the d. of surface traps by 82%, thereby enhancing the charge cond.
- 55Joyce, H. J.; Wong-Leung, J.; Yong, C.; Docherty, C. J.; Paiman, S.; Gao, Q.; Tan, H. H.; Jagadish, C.; Lloyd-Hughes, J.; Herz, L. M.et al. Ultra-low Surface Recombination Velocity in InP Nanowires Probed by Terahertz Spectroscopy Nano Lett. 2012, 12, 5325– 5330There is no corresponding record for this reference.
- 56Joyce, H. J.; Docherty, C. J.; Gao, Q.; Tan, H. H.; Jagadish, C.; Lloyd-Hughes, J.; Herz, L. M.; Johnston, M. B. Electronic Properties of GaAs, Inas and InP Nanowires Studied by Terahertz Spectroscopy Nanotechnology 2013, 24, 21400656https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXht1ajt7zP&md5=147d13a2eeb16bdd25555aae6f47e6afElectronic properties of GaAs, InAs and InP nanowires studied by terahertz spectroscopyJoyce, Hannah J.; Docherty, Callum J.; Gao, Qiang; Tan, H. Hoe; Jagadish, Chennupati; Lloyd-Hughes, James; Herz, Laura M.; Johnston, Michael B.Nanotechnology (2013), 24 (21), 214006/1-214006/7, 7 pp.CODEN: NNOTER; ISSN:1361-6528. (IOP Publishing Ltd.)We have performed a comparative study of ultrafast charge carrier dynamics in a range of III-V nanowires using optical pump-terahertz probe spectroscopy. This versatile technique allows measurement of important parameters for device applications, including carrier lifetimes, surface recombination velocities, carrier mobilities and donor doping levels. GaAs, InAs and InP nanowires of varying diams. were measured. For all samples, the electronic response was dominated by a pronounced surface plasmon mode. Of the three nanowire materials, InAs nanowires exhibited the highest electron mobilities of 6000 cm2 V-1 s-1, which highlights their potential for high mobility applications, such as field effect transistors. InP nanowires exhibited the longest carrier lifetimes and the lowest surface recombination velocity of 170 cm s-1. This very low surface recombination velocity makes InP nanowires suitable for applications where carrier lifetime is crucial, such as in photovoltaics. In contrast, the carrier lifetimes in GaAs nanowires were extremely short, of the order of picoseconds, due to the high surface recombination velocity, which was measured as 5.4 × 105 cm s-1. These findings will assist in the choice of nanowires for different applications, and identify the challenges in producing nanowires suitable for future electronic and optoelectronic devices.
- 57Fortin, E.; Raga, F. Excitons in Molybdenum-disulfide Phys. Rev. B: Solid State 1975, 11, 905– 91257https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaE2MXhtlWntbo%253D&md5=f139eb8016fd2fe2b2e68b0cc235e5bfExcitons in molybdenum disulfideFortin, Emery; Raga, FrancescoPhysical Review B: Solid State (1975), 11 (2), 905-12CODEN: PLRBAQ; ISSN:0556-2805.Optical spectra of MoS2 in the excitonic region were obtained by photocond., photovoltaic effect and wavelength-modulated reflectivity at temps. of 300 to 4.2°K and in magnetic fields of up to 70 kG. Cleaved natural crystals and synthetic crystals studied in both the Faraday and Voigt configurations showed nearly the same characteristics. The results indicate the possibility of up to 4 excitonic series in MoS2 as in other Mo dichalcogenides, and support band-structure calcns. predicting flat conduction and valence bands originating from Mo orbitals. A ground-state anomaly of the A exciton is explained by a central cell correction of the type used by G. Harbeke and E. Tosatti (1972) to explain a similar anomaly in PbI2. The reduced mass of the A exciton is μ* ≃ 0.4m0, both from the study of the series itself, and from its magnetooptical properties, thus removing a large discrepancy found in previous work.
- 58Yong, C. K.; Joyce, H. J.; Lloyd-Hughes, J.; Gao, Q.; Tan, H. H.; Jagadish, C.; Johnston, M. B.; Herz, L. M. Ultrafast Dynamics of Exciton Formation In Semiconductor Nanowires Small 2012, 8, 1725– 1731There is no corresponding record for this reference.
- 59Docherty, C. J.; Lin, C.; Joyce, H. J.; Nicholas, R. J.; Herz, L. M.; Li, L.; Johnston, M. B. Extreme Sensitivity of Graphene Photoconductivity To Environmental Gases Nat. Commun. 2012, 3, 122859https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BC3s7mvFajuw%253D%253D&md5=98eaf4df586f605dd39c7b69749a7cf3Extreme sensitivity of graphene photoconductivity to environmental gasesDocherty Callum J; Lin Cheng-Te; Joyce Hannah J; Nicholas Robin J; Herz Laura M; Li Lain-Jong; Johnston Michael BNature communications (2012), 3 (), 1228 ISSN:.Graphene is a single layer of covalently bonded carbon atoms, which was discovered only 8 years ago and yet has already attracted intense research and commercial interest. Initial research focused on its remarkable electronic properties, such as the observation of massless Dirac fermions and the half-integer quantum Hall effect. Now graphene is finding application in touch-screen displays, as channels in high-frequency transistors and in graphene-based integrated circuits. The potential for using the unique properties of graphene in terahertz-frequency electronics is particularly exciting; however, initial experiments probing the terahertz-frequency response of graphene are only just emerging. Here we show that the photoconductivity of graphene at terahertz frequencies is dramatically altered by the adsorption of atmospheric gases, such as nitrogen and oxygen. Furthermore, we observe the signature of terahertz stimulated emission from gas-adsorbed graphene. Our findings highlight the importance of environmental conditions on the design and fabrication of high-speed, graphene-based devices.
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detailing the pump fluence and pump energy dependence of the THz spectroscopy measurements, as well as information above removing reflections from the trilayer THz photoconductivity. This material is available free of charge via the Internet at http://pubs.acs.org.
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