@article{su_yu_cao_zhang_2023, title={Correlative spectroscopic investigations of the mechanisms of inhomogeneity in CVD-grown monolayer WS<sub>2</sub>}, ISSN={["2199-4501"]}, DOI={10.1007/s40843-023-2616-x}, abstractNote={Chemical vapor deposition (CVD) has been proved to be the most useful method to produce two-dimensional (2D) materials, including tungsten disulfide (WS2). However, the existence of inhomogeneity of strain, doping, and defects in the CVD-grown WS2 monolayers may significantly influence the optical and electronic properties of the materials, thus affecting their device applications. In this work, we systematically characterized the inhomogeneity of strain, doping, and nonradiative defect centers in mesoscopic-size, triangular-shape monolayer WS2 grown by CVD on sapphire substrate by using spatially resolved micro-Raman and photoluminescence (PL) spectroscopy. We performed correlative analyses on the variations of the pertinent spectral parameters (i.e., peak position, intensity, and full width at half maximum) of Raman and PL signatures in two physical scales: (1) the complete-data-set level, including the data of the whole sample, and (2) the sub-data-set level for individual special regions (e.g., apexes, edges, center) that exhibit distinctly different strain, doping, and defect states. This study reveals and explains the inhomogeneous strain, doping, and defects across the WS2 monolayer. Additionally, we find the inhomogeneity substantially diminishes when a mesoscopic-size triangle structure expands into a continuous film. Our work demonstrates that the correlative analyses, supported with physics insights, can offer comprehensive understanding on the underlying mechanisms of the inhomogeneity and guidance for optimizing the growth process and device processing of 2D materials.}, journal={SCIENCE CHINA-MATERIALS}, author={Su, Liqin and Yu, Yifei and Cao, Linyou and Zhang, Yong}, year={2023}, month={Sep} }
 @article{cheng_backman_zhang_abuzaid_li_yu_cao_davydov_luisier_richter_et al._2023, title={Distinct Contact Scaling Effects in MoS2 Transistors Revealed with Asymmetrical Contact Measurements}, ISSN={["1521-4095"]}, DOI={10.1002/adma.202210916}, abstractNote={2D semiconducting materials have immense potential for future electronics due to their atomically thin nature, which enables better scalability. While the channel scalability of 2D materials has been extensively studied, the current understanding of contact scaling in 2D devices is inconsistent and oversimplified. Here physically scaled contacts and asymmetrical contact measurements (ACMs) are combined to investigate the contact scaling behavior in 2D field‐effect transistors. The ACMs directly compare electron injection at different contact lengths while using the exact same MoS2 channel, eliminating channel‐to‐channel variations. The results show that scaled source contacts can limit the drain current, whereas scaled drain contacts do not. Compared to devices with long contact lengths, devices with short contact lengths (scaled contacts) exhibit larger variations, 15% lower drain currents at high drain–source voltages, and a higher chance of early saturation and negative differential resistance. Quantum transport simulations reveal that the transfer length of Ni–MoS2 contacts can be as short as 5 nm. Furthermore, it is clearly identified that the actual transfer length depends on the quality of the metal‐2D interface. The ACMs demonstrated here will enable further understanding of contact scaling behavior at various interfaces.}, journal={ADVANCED MATERIALS}, author={Cheng, Zhihui and Backman, Jonathan and Zhang, Huairuo and Abuzaid, Hattan and Li, Guoqing and Yu, Yifei and Cao, Linyou and Davydov, Albert V. and Luisier, Mathieu and Richter, Curt A. and et al.}, year={2023}, month={Apr} }
 @article{aslan_yule_yu_lee_heinz_cao_brongersma_2022, title={Excitons in strained and suspended monolayer WSe2}, volume={9}, ISSN={["2053-1583"]}, DOI={10.1088/2053-1583/ac2d15}, abstractNote={We study suspended membranes of atomically thin WSe2 as hosts of excitons. We perform optical reflectance measurements to probe the exciton physics and obtain the peak energies for the 1 s , 2 s , and 3 s states of the A exciton in suspended WSe2 and consider supported membranes as a reference. We find that elimination of the influence of the dielectric environment enables a strong electron–hole interaction and a concomitant increase in the exciton binding energy in suspended monolayer (1L) WSe2. Based on the experimental results, we calculate the excitonic binding energies by employing the recently developed quantum electrostatic heterostructure model and the commonly employed Rytova–Keldysh potential model. We see that the binding energy of the ground state A exciton increases from about 0.3 eV (on a substrate) to above 0.4 eV (suspended). We also exploit the tunability of the excitons in suspended samples via mechanical strain. By applying external gas pressure of 2.72 atm to a 1L suspended over a circular hole of 8 μm diameter, we strain the WSe2 and obtain a reversible 0.15 eV redshift in the exciton resonance. The linewidth of the A exciton decreases by more than half, from about 50 to 20 meV under 1.5% biaxial strain at room temperature. This line narrowing is due to the suppression of intervalley exciton–phonon scattering. By making use of the observed strain-dependent optical signatures, we infer the two-dimensional (2D) elastic moduli of 1L and 2L WSe2. Our results exemplify the use of suspended 2D materials as novel systems for fundamental studies, as well as for strong and dynamic tuning of their optical properties.}, number={1}, journal={2D MATERIALS}, author={Aslan, Burak and Yule, Colin and Yu, Yifei and Lee, Yan Joe and Heinz, Tony F. and Cao, Linyou and Brongersma, Mark L.}, year={2022}, month={Jan} }
 @article{yu_yu_li_puretzky_geohegan_cao_2020, title={Giant enhancement of exciton diffusivity in two-dimensional semiconductors}, volume={6}, ISSN={["2375-2548"]}, DOI={10.1126/sciadv.abb4823}, abstractNote={The exciton diffusivity of 2D semiconductors can be improved by 15-fold with trapped charges that can screen exciton scattering.}, number={51}, journal={SCIENCE ADVANCES}, author={Yu, Yiling and Yu, Yifei and Li, Guoqing and Puretzky, Alexander A. and Geohegan, David B. and Cao, Linyou}, year={2020}, month={Dec} }
 @article{yu_minhaj_huang_yu_cao_2020, title={In-Plane and Interfacial Thermal Conduction of Two-Dimensional Transition-Metal Dichalcogenides}, volume={13}, ISSN={["2331-7019"]}, DOI={10.1103/PhysRevApplied.13.034059}, abstractNote={We elucidate the dependence of the in-plane and interfacial thermal conduction of two-dimensional (2D) transition-metal dichalcogenide (TMDC) materials (including ${\mathrm{Mo}\mathrm{S}}_{2}$, ${\mathrm{WS}}_{2}$, and ${\mathrm{W}\mathrm{Se}}_{2}$) on the materials' physical features, such as size, layer number, composition, and substrates. The in-plane thermal conductivity k is measured at suspended 2D TMDC materials and the interfacial thermal conductance g is measured at materials supported on substrates, both through Raman thermometry techniques. The thermal conductivity k increases with the radius R of the suspended area following a logarithmic scaling as k\ensuremath{\sim}log(R). k also shows a substantial decrease from monolayer to bilayer, but only changes slightly with a further increase in the layer number. In contrast, the interfacial thermal conductance g has a negligible dependence on the layer number, but g increases with the strength of the interaction between 2D TMDC materials and the substrate, substantially varying among different substrates. The result is consistent with theoretical predictions and clarifies much inconsistence in the literature. This work provides useful guidance for thermal management in 2D TMDC materials and devices.}, number={3}, journal={PHYSICAL REVIEW APPLIED}, author={Yu, Yifei and Minhaj, Tamzid and Huang, Lujun and Yu, Yiling and Cao, Linyou}, year={2020}, month={Mar} }
 @article{cheng_abuzaid_yu_zhang_li_noyce_williams_lin_doherty_tao_et al._2019, title={Convergent ion beam alteration of 2D materials and metal-2D interfaces}, volume={6}, ISSN={["2053-1583"]}, DOI={10.1088/2053-1583/ab1764}, abstractNote={Tailoring the properties of two-dimensional (2D) crystals is important for both understanding the material behavior and exploring new functionality. Here we demonstrate the alteration of MoS2 and metal-MoS2 interfaces using a convergent ion beam. Different beam energies, from 60 eV to 600 eV, are shown to have distinct effects on the optical and electrical properties of MoS2. Defects and deformations created across different layers were investigated, revealing an unanticipated improvement in the Raman peak intensity of multilayer MoS2 when exposed to a 60 eV Ar+ ion beam, and attenuation of the MoS2 Raman peaks with a 200 eV ion beam. Using cross-sectional scanning transmission electron microscopy (STEM), alteration of the crystal structure after a 600 eV ion beam bombardment was observed, including generated defects and voids in the crystal. We show that the 60 eV ion beam yields improvement in the metal-MoS2 interface by decreasing the contact resistance from 17.5 kΩ · µm to 6 kΩ · µm at a carrier concentration of n2D  =  5.4  ×  1012 cm−2. These results advance the use of low-energy ion beams to modify 2D materials and interfaces for tuning and improving performance in applications of sensors, transistors, optoelectronics, and so forth.}, number={3}, journal={2D MATERIALS}, author={Cheng, Zhihui and Abuzaid, Hattan and Yu, Yifei and Zhang, Fan and Li, Yanlong and Noyce, Steven G. and Williams, Nicholas X. and Lin, Yuh-Chen and Doherty, James L. and Tao, Chenggang and et al.}, year={2019}, month={Jul} }
 @article{cheng_yu_singh_price_noyce_lin_cao_franklin_2019, title={Immunity to Contact Scaling in MoS2 Transistors Using in Situ Edge Contacts}, volume={19}, ISSN={["1530-6992"]}, DOI={10.1021/acs.nanolett.9b01355}, abstractNote={Atomically thin two-dimensional (2D) materials are promising candidates for sub-10 nm transistor channels due to their ultrathin body thickness, which results in strong electrostatic gate control. Properly scaling a transistor technology requires reducing both the channel length (distance from source to drain) and the contact length (distance that source and drain interface with semiconducting channel). Contact length scaling remains an unresolved epidemic for transistor scaling, affecting devices from all semiconductors-silicon to 2D materials. Here, we show that clean edge contacts to 2D MoS2 can provide immunity to the contact-scaling problem, with performance that is independent of contact length down to the 20 nm regime. Using a directional ion beam, in situ edge contacts of various metal-MoS2 interfaces are studied. Characterization of the intricate edge interface using cross-sectional electron microscopy reveals distinct morphological effects on the MoS2 depending on its thickness-from monolayer to few-layer films. The in situ edge contacts also exhibit an order of magnitude higher performance compared to the best-reported ex situ metal edge contacts. Our work provides experimental evidence for a solution to contact scaling in transistors, using 2D materials with clean edge contact interfaces, opening a new way of designing devices with 2D materials.}, number={8}, journal={NANO LETTERS}, author={Cheng, Zhihui and Yu, Yifei and Singh, Shreya and Price, Katherine and Noyce, Steven G. and Lin, Yuh-Chen and Cao, Linyou and Franklin, Aaron D.}, year={2019}, month={Aug}, pages={5077–5085} }
 @article{yu_bataller_younts_yu_li_puretzky_geohegan_gundogdu_cao_2019, title={Room-Temperature Electron-Hole Liquid in Monolayer MoS2}, volume={13}, ISSN={["1936-086X"]}, DOI={10.1021/acsnano.9b04124}, abstractNote={Excitons in semiconductors are usually non-interacting and behave like an ideal gas, but may condense to a strongly-correlated liquid-like state, i.e. electron-hole liquid (EHL), at high density and appropriate temperature. EHL is a macroscopic quantum state with exotic properties and represents the ultimate attainable charge excitation density in steady states. It bears great promise for a variety of fields such as ultrahigh-power photonics and quantum science and technology. However, the condensation of gas-like excitons to EHL has often been restricted to cryogenic temperatures, which significantly limits the prospect of EHL for use in practical applications. Herein we demonstrate the formation of EHL at room temperature in monolayer MoS2 by taking advantage of the monolayer's extraordinarily strong exciton binding energy. This work demonstrates the potential for the liquid-like state of charge excitations to be a useful platform for the studies of macroscopic quantum phenomena and the development of optoelectronic devices.}, number={9}, journal={ACS NANO}, author={Yu, Yiling and Bataller, Alexander W. and Younts, Robert and Yu, Yifei and Li, Guoqing and Puretzky, Alexander A. and Geohegan, David B. and Gundogdu, Kenan and Cao, Linyou}, year={2019}, month={Sep}, pages={10351–10358} }
 @article{su_bradley_yu_yu_cao_zhao_zhang_2019, title={Surface-enhanced Raman scattering of monolayer transition metal dichalcogenides on Ag nanorod arrays}, volume={44}, ISSN={["1539-4794"]}, DOI={10.1364/OL.44.005493}, abstractNote={In this work, we studied surface-enhanced Raman scattering (SERS) of MS2 (M=Mo, W) monolayers that were transferred onto Ag nanorod arrays. Compared to the suspended monolayers, the Raman intensity of monolayers on an Ag nanorod substrate was strongly enhanced for both in-plane and out-of-plane vibration modes: up to 8 (5) for E2g and 20 (23) for A1g in MoS2 (WS2). This finding reveals a promising SERS substrate for achieving uniform and strong enhancement for two-dimensional materials in the applications of optical detecting and sensing.}, number={22}, journal={OPTICS LETTERS}, author={Su, Liqin and Bradley, Layne and Yu, Yiling and Yu, Yifei and Cao, Linyou and Zhao, Yiping and Zhang, Yong}, year={2019}, month={Nov}, pages={5493–5496} }
 @article{li_zhang_yu_huang_yang_cao_2017, title={Activating MoS2 for pH-Universal Hydrogen Evolution Catalysis}, volume={139}, ISSN={["1520-5126"]}, DOI={10.1021/jacs.7b07450}, abstractNote={MoS2 presents a promising catalyst for the hydrogen evolution reaction (HER) in water splitting, but its worse catalytic performance in neutral and alkaline media than in acidic environment may be problematic for practical application. This is because the other half reaction of water splitting, i.e., oxygen evolution reaction, often needs to be implemented in alkaline environment. Here we demonstrate a universal strategy that may be used to significantly improve the HER catalysis of MoS2 in all kinds of environments from acidic to alkaline, proton intercalation. Protons may be enabled to intercalate between monolayer MoS2 and underlying substrates or in the interlayer space of thicker MoS2 by two processes: electrochemically polarizing MoS2 at negative potentials (vs RHE) in acidic media or immersing MoS2 into certain acid solutions like TFSI. The improvement in catalytic performance is due to the activity enhancement of the active sites in MoS2 by the intercalated protons, which might be related with the effect of the intercalated protons on electrical conductance and the adsorption energy of hydrogen atoms. The enhancement in catalytic activity by the intercalated proton is very stable even in neutral and alkaline electrolytes.}, number={45}, journal={JOURNAL OF THE AMERICAN CHEMICAL SOCIETY}, author={Li, Guoqing and Zhang, Du and Yu, Yifei and Huang, Shengyang and Yang, Weitao and Cao, Linyou}, year={2017}, month={Nov}, pages={16194–16200} }
 @article{yu_li_huang_barrette_cai_yu_gundogdu_zhang_cao_2017, title={Enhancing Multifunctionalities of Transition-Metal Dichalcogenide Monolayers via Cation Intercalation}, volume={11}, ISSN={1936-0851 1936-086X}, url={http://dx.doi.org/10.1021/ACSNANO.7B04880}, DOI={10.1021/acsnano.7b04880}, abstractNote={We have demonstrated that multiple functionalities of transition-metal dichalcogenide (TMDC) monolayers may be substantially improved by the intercalation of small cations (H+ or Li+) between the monolayers and underlying substrates. The functionalities include photoluminescence (PL) efficiency and catalytic activity. The improvement in PL efficiency may be up to orders of magnitude and can be mainly ascribed to two effects of the intercalated cations: p-doping to the monolayers and reducing the influence of substrates, but more studies are necessary to better understand the mechanism for the improvement in the catalytic functionality. The cation intercalation may be achieved by simply immersing substrate-supported monolayers into the solution of certain acids or salts. It is more difficult to intercalate under the monolayers interacting with substrates stronger, such as as-grown monolayers or the monolayers on 2D material substrates. This result presents a versatile strategy to simultaneously optimize multiple functionalities of TMDC monolayers.}, number={9}, journal={ACS Nano}, publisher={American Chemical Society (ACS)}, author={Yu, Yifei and Li, Guoqing and Huang, Lujun and Barrette, Andrew and Cai, Yong-Qing and Yu, Yiling and Gundogdu, Kenan and Zhang, Yong-Wei and Cao, Linyou}, year={2017}, month={Sep}, pages={9390–9396} }
 @article{yu_yu_huang_peng_xiong_cao_2017, title={Giant Gating Tunability of Optical Refractive Index in Transition Metal Dichalcogenide Monolayers}, volume={17}, ISSN={["1530-6992"]}, DOI={10.1021/acs.nanolett.7b00768}, abstractNote={We report that the refractive index of transition metal dichacolgenide (TMDC) monolayers, such as MoS2, WS2, and WSe2, can be substantially tuned by >60% in the imaginary part and >20% in the real part around exciton resonances using complementary metal-oxide-semiconductor (CMOS) compatible electrical gating. This giant tunablility is rooted in the dominance of excitonic effects in the refractive index of the monolayers and the strong susceptibility of the excitons to the influence of injected charge carriers. The tunability mainly results from the effects of injected charge carriers to broaden the spectral width of excitonic interband transitions and to facilitate the interconversion of neutral and charged excitons. The other effects of the injected charge carriers, such as renormalizing bandgap and changing exciton binding energy, only play negligible roles. We also demonstrate that the atomically thin monolayers, when combined with photonic structures, can enable the efficiencies of optical absorption (reflection) tuned from 40% (60%) to 80% (20%) due to the giant tunability of the refractive index. This work may pave the way toward the development of field-effect photonics in which the optical functionality can be controlled with CMOS circuits.}, number={6}, journal={NANO LETTERS}, author={Yu, Yiling and Yu, Yifei and Huang, Lujun and Peng, Haowei and Xiong, Liwei and Cao, Linyou}, year={2017}, month={Jun}, pages={3613–3618} }
 @article{su_yu_cao_zhang_2017, title={In Situ Monitoring of the Thermal-Annealing Effect in a Monolayer of MoS2}, volume={7}, ISSN={["2331-7019"]}, DOI={10.1103/physrevapplied.7.034009}, abstractNote={We perform in situ two-cycle thermal-annealing studies for a transferred CVD-grown monolayer MoS2 on a SiO2=Si substrate, using spatially resolved micro-Raman and photoluminescence spectroscopy. The evolution in film morphology and film-substrate bonding is continuously monitored by Raman spectroscopy. After the thermal cycling and being annealed at 305 °C twice, the film morphology and filmsubstrate bonding are significantly modified, which together with the removal of polymer residues causes major changes in the strain and doping distribution over the film, and thus the optical properties. Before annealing, the strain associated with ripples in the transferred film dominates the spatial distributions of the photoluminescence peak position and intensity over the film; after annealing, the variation in film-substrate bonding, affecting both strain and doping, becomes the leading factor. This work reveals that the filmsubstrate bonding, and thus the strain and doping, is nonstationary under thermal stress, which is important for understanding the substrate effects on the optical and transport properties of the 2D material and their impact on device applications.}, number={3}, journal={PHYSICAL REVIEW APPLIED}, author={Su, Liqin and Yu, Yifei and Cao, Linyou and Zhang, Yong}, year={2017}, month={Mar} }
 @article{li_zhang_qiao_yu_peterson_zafar_kumar_curtarolo_hunte_shannon_et al._2016, title={All The Catalytic Active Sites of MoS2 for Hydrogen Evolution}, volume={138}, ISSN={0002-7863 1520-5126}, url={http://dx.doi.org/10.1021/jacs.6b05940}, DOI={10.1021/jacs.6b05940}, abstractNote={MoS2 presents a promising low-cost catalyst for the hydrogen evolution reaction (HER), but the understanding about its active sites has remained limited. Here we present an unambiguous study of the catalytic activities of all possible reaction sites of MoS2, including edge sites, sulfur vacancies, and grain boundaries. We demonstrate that, in addition to the well-known catalytically active edge sites, sulfur vacancies provide another major active site for the HER, while the catalytic activity of grain boundaries is much weaker. The intrinsic turnover frequencies (Tafel slopes) of the edge sites, sulfur vacancies, and grain boundaries are estimated to be 7.5 s-1 (65-75 mV/dec), 3.2 s-1 (65-85 mV/dec), and 0.1 s-1 (120-160 mV/dec), respectively. We also demonstrate that the catalytic activity of sulfur vacancies strongly depends on the density of the vacancies and the local crystalline structure in proximity to the vacancies. Unlike edge sites, whose catalytic activity linearly depends on the length, sulfur vacancies show optimal catalytic activities when the vacancy density is in the range of 7-10%, and the number of sulfur vacancies in high crystalline quality MoS2 is higher than that in low crystalline quality MoS2, which may be related with the proximity of different local crystalline structures to the vacancies.}, number={51}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Li, Guoqing and Zhang, Du and Qiao, Qiao and Yu, Yifei and Peterson, David and Zafar, Abdullah and Kumar, Raj and Curtarolo, Stefano and Hunte, Frank and Shannon, Steve and et al.}, year={2016}, month={Dec}, pages={16632–16638} }
 @article{yu_yu_xu_cai_su_zhang_zhang_gundogdu_cao_2016, title={Engineering Substrate Interactions for High Luminescence Efficiency of Transition-Metal Dichalcogenide Monolayers}, volume={26}, ISSN={["1616-3028"]}, DOI={10.1002/adfm.201600418}, abstractNote={It is demonstrated that the luminescence efficiency of monolayers composed of MoS2, WS2, and WSe2 is significantly limited by the substrate and can be improved by orders of magnitude through substrate engineering. The substrate affects the efficiency mainly through doping the monolayers and facilitating defect‐assisted nonradiative exciton recombinations, while the other substrate effects including straining and dielectric screening play minor roles. The doping may come from the substrate and substrate‐borne water moisture, the latter of which is much stronger than the former for MoS2 and WS2 but negligible for WSe2. Using proper substrates such as mica or hexagonal boron nitride can substantially mitigate the doping effect. The defect‐assisted recombination depends on the interaction between the defect in the monolayer and the substrate. Suspended monolayers, in which the substrate effects are eliminated, may have efficiency up to 40% at room temperatures. The result provides useful guidance for the rational design of atomic‐scale light emission devices.}, number={26}, journal={ADVANCED FUNCTIONAL MATERIALS}, author={Yu, Yifei and Yu, Yiling and Xu, Chao and Cai, Yong-Qing and Su, Liqin and Zhang, Yong and Zhang, Yong-Wei and Gundogdu, Kenan and Cao, Linyou}, year={2016}, month={Jul}, pages={4733–4739} }
 @article{yu_yu_xu_barrette_gundogdu_cao_2016, title={Fundamental limits of exciton-exciton annihilation for light emission in transition metal dichalcogenide monolayers}, volume={93}, DOI={10.1103/physrevb.93.201111}, abstractNote={We quantitatively evaluate the exciton-exciton annihilation (EEA) and its effect on light emission properties in monolayer TMDC materials, including WS2, MoS2, and WSe2. The EEA rate is found to be 0.3 cm2/s and 0.1 cm2/s for suspended WS2 and MoS2 monolayers, respectively, and subject to the influence from substrates, being 0.1 cm2/s and 0.05 cm2/s for the supported WS2 and MoS2 on sapphire substrates. It can substantially affect the luminescence efficiency of suspended monolayers even at an exciton concentration as low as 109 cm-2, but plays a milder role for supported monolayers due to the effect of the substrate. However, regardless the presence of substrates or not, the lasing threshold of the monolayer is always predominantly determined by the EEA, which is estimated to be 12-18 MW/cm2 if using 532 nm as the pumping wavelength.}, number={20}, journal={Physical Review B}, author={Yu, Y. L. and Yu, Y. F. and Xu, C. and Barrette, A. and gundogdu and Cao, L. Y.}, year={2016} }
 @article{mills_yu_chen_huang_cao_tao_2016, title={Ripples near edge terminals in MoS2 few layers and pyramid nanostructures}, volume={108}, ISSN={["1077-3118"]}, DOI={10.1063/1.4942088}, abstractNote={Atomically thin transition-metal dichalcogenides are of great interest due to their intriguing physical properties and potential applications. Here, we report our findings from scanning tunneling microscopy and spectroscopy investigations on molybdenum disulfide (MoS2) mono- to few-layers and pyramid nanostructures synthesized through chemical vapor deposition. On the few-layered MoS2 nanoplatelets grown on gallium nitride (GaN) and pyramid nanostructures on highly oriented pyrolytic graphite, we observed an intriguing curved region near the edge terminals. The measured band gap on these curved regions is 1.96 ± 0.10 eV, consistent with the value of the direct band gap in MoS2 monolayers. The curved features near the edge terminals and the associated electronic properties may contribute to the catalytic behaviors of MoS2 nanostructures and have potential applications in future electronic devices and energy-related products based on MoS2 nanostructures.}, number={8}, journal={APPLIED PHYSICS LETTERS}, author={Mills, Adam and Yu, Yifei and Chen, Chuanhui and Huang, Bevin and Cao, Linyou and Tao, Chenggang}, year={2016}, month={Feb} }
 @article{su_yu_cao_zhang_2015, title={Effects of substrate type and material-substrate bonding on high-temperature behavior of monolayer WS2}, volume={8}, ISSN={["1998-0000"]}, DOI={10.1007/s12274-015-0775-1}, number={8}, journal={NANO RESEARCH}, author={Su, Liqin and Yu, Yifei and Cao, Linyou and Zhang, Yong}, year={2015}, month={Aug}, pages={2686–2697} }
 @article{li_yu_huang_nielsen_goddard_li_cao_2015, title={Engineering the Composition and Crystallinity of Molybdenum Sulfide for High-Performance Electrocatalytic Hydrogen Evolution}, volume={5}, ISSN={["2155-5435"]}, DOI={10.1021/cs501635v}, abstractNote={The key challenge for the development of high-performance molybdenum sulfide HER catalysts lies in the limited fundamental understanding for the correlation between the catalytic activities and physical features of the materials. Here we have demonstrated an unambiguous correlation between the catalytic performance and the composition/crystallinity of molybdenum sulfide. The results indicate that the crystallinity plays an overwhelming role in determining the catalytic performance, while the composition does not matter much. The crystallinity can affect the three figures of merit of the catalytic performance (Tafel slope, turnover frequency (TOF), and stability) in opposite directions. Generally, the materials with low crystalline quality may provide low Tafel slopes (∼40 mV/dec), while highly crystalline molybdenum sulfide shows higher TOFs (by 2 orders of magnitude) and better stability. DFT calculations suggest that the terminal disulfur complex S22–, which may exist in MoS3 and also likely MoS2 of low...}, number={1}, journal={ACS CATALYSIS}, author={Li, Yanpeng and Yu, Yifei and Huang, Yufeng and Nielsen, Robert A. and Goddard, William A., III and Li, Yao and Cao, Linyou}, year={2015}, month={Jan}, pages={448–455} }
 @article{yu_yu_cai_li_gurarslan_peelaers_aspnes_walle_nguyen_zhang_et al._2015, title={Exciton-dominated Dielectric Function of Atomically Thin MoS2 Films}, volume={5}, ISSN={["2045-2322"]}, DOI={10.1038/srep16996}, abstractNote={We systematically measure the dielectric function of atomically thin MoS2 films with different layer numbers and demonstrate that excitonic effects play a dominant role in the dielectric function when the films are less than 5-7 layers thick. The dielectric function shows an anomalous dependence on the layer number. It decreases with the layer number increasing when the films are less than 5-7 layers thick but turns to increase with the layer number for thicker films. We show that this is because the excitonic effect is very strong in the thin MoS2 films and its contribution to the dielectric function may dominate over the contribution of the band structure. We also extract the value of layer-dependent exciton binding energy and Bohr radius in the films by fitting the experimental results with an intuitive model. The dominance of excitonic effects is in stark contrast with what reported at conventional materials whose dielectric functions are usually dictated by band structures. The knowledge of the dielectric function may enable capabilities to engineer the light-matter interactions of atomically thin MoS2 films for the development of novel photonic devices, such as metamaterials, waveguides, light absorbers, and light emitters.}, journal={SCIENTIFIC REPORTS}, author={Yu, Yiling and Yu, Yifei and Cai, Yongqing and Li, Wei and Gurarslan, Alper and Peelaers, Hartwin and Aspnes, David E. and Walle, Chris G. and Nguyen, Nhan V. and Zhang, Yong-Wei and et al.}, year={2015}, month={Nov} }
 @article{su_zhang_yu_cao_2014, title={Dependence of coupling of quasi 2-D MoS2 with substrates on substrate types, probed by temperature dependent Raman scattering}, volume={6}, ISSN={["2040-3372"]}, DOI={10.1039/c3nr06462j}, abstractNote={This work reports a study on the temperature dependence of in-plane E and out-of-plane A1g Raman modes of single-layer (1L) and bi-layer (2L) MoS2 films on sapphire (epitaxial) and SiO2 (transferred) substrates as well as bulk MoS2 single crystals in a temperature range of 25-500 °C. For the films on the transferred SiO2 substrate, the in-plane E mode is only weakly affected by the substrate, whereas the out-of-plane A1g mode is strongly perturbed, showing highly nonlinear, sometimes even non-monotonic, temperature dependence on the Raman peak shift and linewidth. In contrast, for the films on the epitaxial sapphire substrate, E is affected more significantly by the substrate than A1g. This study suggests that the 2-D film-substrate coupling depends sensitively on the preparation method, and in particular on the film morphology for the transferred film. These findings are vitally important for the fundamental understanding and application of this quasi 2-D material that is expected to be supported by a substrate in most circumstances.}, number={9}, journal={NANOSCALE}, author={Su, Liqin and Zhang, Yong and Yu, Yifei and Cao, Linyou}, year={2014}, pages={4920–4927} }
 @article{yu_hu_su_huang_liu_jin_purezky_geohegan_kim_zhang_et al._2014, title={Equally Efficient Interlayer Exciton Relaxation and Improved Absorption in Epitaxial and Nonepitaxial MoS2/WS2 Heterostructures}, volume={15}, ISSN={1530-6984 1530-6992}, url={http://dx.doi.org/10.1021/nl5038177}, DOI={10.1021/nl5038177}, abstractNote={Semiconductor heterostructures provide a powerful platform to engineer the dynamics of excitons for fundamental and applied interests. However, the functionality of conventional semiconductor heterostructures is often limited by inefficient charge transfer across interfaces due to the interfacial imperfection caused by lattice mismatch. Here we demonstrate that MoS(2)/WS(2) heterostructures consisting of monolayer MoS(2) and WS(2) stacked in the vertical direction can enable equally efficient interlayer exciton relaxation regardless the epitaxy and orientation of the stacking. This is manifested by a similar 2 orders of magnitude decrease of photoluminescence intensity in both epitaxial and nonepitaxial MoS(2)/WS(2) heterostructures. Both heterostructures also show similarly improved absorption beyond the simple superimposition of the absorptions of monolayer MoS(2) and WS(2). Our result indicates that 2D heterostructures bear significant implications for the development of photonic devices, in particular those requesting efficient exciton separation and strong light absorption, such as solar cells, photodetectors, modulators, and photocatalysts. It also suggests that the simple stacking of dissimilar 2D materials with random orientations is a viable strategy to fabricate complex functional 2D heterostructures, which would show similar optical functionality as the counterpart with perfect epitaxy.}, number={1}, journal={Nano Letters}, publisher={American Chemical Society (ACS)}, author={Yu, Yifei and Hu, Shi and Su, Liqin and Huang, Lujun and Liu, Yi and Jin, Zhenghe and Purezky, Alexander A. and Geohegan, David B. and Kim, Ki Wook and Zhang, Yong and et al.}, year={2014}, month={Dec}, pages={486–491} }
 @article{mai_semenov_barrette_yu_jin_cao_kim_gundogdu_2014, title={Exciton valley relaxation in a single layer ofWS2measured by ultrafast spectroscopy}, volume={90}, ISSN={1098-0121 1550-235X}, url={http://dx.doi.org/10.1103/PhysRevB.90.041414}, DOI={10.1103/physrevb.90.041414}, abstractNote={We measured the lifetime of optically created valley polarization in single layer WS2 using transient absorption spectroscopy. The electron valley relaxation is very short (< 1ps). However the hole valley lifetime is at least two orders of magnitude longer and exhibits a temperature dependence that cannot be explained by single carrier spin/valley relaxation mechanisms. Our theoretical analysis suggests that a collective contribution of two potential processes may explain the valley relaxation in single layer WS2. One process involves direct scattering of excitons from K to K' valleys with a spin flip-flop interaction. The other mechanism involves scattering through spin degenerate Gamma valley. This second process is thermally activated with an Arrhenius behavior due to the energy barrier between Gamma and K valleys.}, number={4}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Mai, Cong and Semenov, Yuriy G. and Barrette, Andrew and Yu, Yifei and Jin, Zhenghe and Cao, Linyou and Kim, Ki Wook and Gundogdu, Kenan}, year={2014}, month={Jul} }
 @article{yu_huang_li_steinmann_yang_cao_2014, title={Layer-Dependent Electrocatalysis of MoS2 for Hydrogen Evolution}, volume={14}, ISSN={["1530-6992"]}, DOI={10.1021/nl403620g}, abstractNote={The quantitative correlation of the catalytic activity with the microscopic structure of heterogeneous catalysts is a major challenge for the field of catalysis science. It requests synergistic capabilities to tailor the structure with atomic scale precision and to control the catalytic reaction to proceed through well-defined pathways. Here we leverage on the controlled growth of MoS2 atomically thin films to demonstrate that the catalytic activity of MoS2 for the hydrogen evolution reaction decreases by a factor of ∼ 4.47 for the addition of every one more layer. Similar layer dependence is also found in edge-riched MoS2 pyramid platelets. This layer-dependent electrocatalysis can be correlated to the hopping of electrons in the vertical direction of MoS2 layers over an interlayer potential barrier. Our experimental results suggest the potential barrier to be 0.119 V, consistent with theoretical calculations. Different from the conventional wisdom, which states that the number of edge sites is important, our results suggest that increasing the hopping efficiency of electrons in the vertical direction is a key for the development of high-efficiency two-dimensional material catalysts.}, number={2}, journal={NANO LETTERS}, author={Yu, Yifei and Huang, Sheng-Yang and Li, Yanpeng and Steinmann, Stephan N. and Yang, Weitao and Cao, Linyou}, year={2014}, month={Feb}, pages={553–558} }
 @article{gurarslan_yu_su_yu_suarez_yao_zhu_ozturk_zhang_cao_2014, title={Surface-Energy-Assisted Perfect Transfer of Centimeter-Scale Mono layer and Few-Layer MoS2 Films onto Arbitrary Substrates}, volume={8}, ISSN={["1936-086X"]}, DOI={10.1021/nn5057673}, abstractNote={The transfer of synthesized 2D MoS2 films is important for fundamental and applied research. However, it is problematic to translate the well-established transfer processes for graphene to MoS2 due to different growth mechanisms and surface properties. Here we demonstrate a surface-energy-assisted process that can perfectly transfer centimeter-scale monolayer and few-layer MoS2 films from original growth substrates onto arbitrary substrates with no observable wrinkles, cracks, and polymer residues. The unique strategies used in this process include leveraging the penetration of water between hydrophobic MoS2 films and hydrophilic growth substrates to lift off the films and dry transferring the film after the lift off. This is in stark contrast with the previous transfer process for synthesized MoS2 films, which explores the etching of the growth substrate by hot base solutions to lift off the films. Our transfer process can effectively eliminate the mechanical force caused by bubble generations, the attacks from chemical etchants, and the capillary force induced when transferring the film outside solutions as in the previous transfer process, which consists of the major causes for the previous unsatisfactory transfer. Our transfer process also benefits from using polystyrene (PS), instead of poly(methyl methacrylate) (PMMA) that was widely used previously, as the carrier polymer. PS can form more intimate interaction with MoS2 films than PMMA and is important for maintaining the integrity of the film during the transfer process. This surface-energy-assisted approach can be generally applied to the transfer of other 2D materials, such as WS2.}, number={11}, journal={ACS NANO}, author={Gurarslan, Alper and Yu, Yifei and Su, Liqin and Yu, Yiling and Suarez, Francisco and Yao, Shanshan and Zhu, Yong and Ozturk, Mehmet and Zhang, Yong and Cao, Linyou}, year={2014}, month={Nov}, pages={11522–11528} }
 @article{mannebach_duerloo_pellouchoud_sher_nah_kuo_yu_marshall_cao_reed_et al._2014, title={Ultrafast Electronic and Structural Response of Monolayer MoS2 under Intense Photoexcitation Conditions}, volume={8}, ISSN={["1936-086X"]}, DOI={10.1021/nn5044542}, abstractNote={We report on the dynamical response of single layer transition metal dichalcogenide MoS2 to intense above-bandgap photoexcitation using the nonlinear-optical second order susceptibility as a direct probe of the electronic and structural dynamics. Excitation conditions corresponding to the order of one electron-hole pair per unit cell generate unexpected increases in the second harmonic from monolayer films, occurring on few picosecond time-scales. These large amplitude changes recover on tens of picosecond time-scales and are reversible at megahertz repetition rates with no photoinduced change in lattice symmetry observed despite the extreme excitation conditions.}, number={10}, journal={ACS NANO}, author={Mannebach, Ehren M. and Duerloo, Karel-Alexander N. and Pellouchoud, Lenson A. and Sher, Meng-Ju and Nah, Sanghee and Kuo, Yi-Hong and Yu, Yifei and Marshall, Ann F. and Cao, Linyou and Reed, Evan J. and et al.}, year={2014}, month={Oct}, pages={10734–10742} }
 @article{yu_li_liu_su_zhang_cao_2013, title={Controlled Scalable Synthesis of Uniform, High-Quality Monolayer and Few-layer MoS2 Films}, volume={3}, ISSN={["2045-2322"]}, DOI={10.1038/srep01866}, abstractNote={Two dimensional (2D) materials with a monolayer of atoms represent an ultimate control of material dimension in the vertical direction. Molybdenum sulfide (MoS2) monolayers, with a direct bandgap of 1.8 eV, offer an unprecedented prospect of miniaturizing semiconductor science and technology down to a truly atomic scale. Recent studies have indeed demonstrated the promise of 2D MoS2 in fields including field effect transistors, low power switches, optoelectronics, and spintronics. However, device development with 2D MoS2 has been delayed by the lack of capabilities to produce large-area, uniform, and high-quality MoS2 monolayers. Here we present a self-limiting approach that can grow high quality monolayer and few-layer MoS2 films over an area of centimeters with unprecedented uniformity and controllability. This approach is compatible with the standard fabrication process in semiconductor industry. It paves the way for the development of practical devices with 2D MoS2 and opens up new avenues for fundamental research.}, journal={SCIENTIFIC REPORTS}, author={Yu, Yifei and Li, Chun and Liu, Yi and Su, Liqin and Zhang, Yong and Cao, Linyou}, year={2013}, month={May} }
 @article{mai_barrette_yu_semenov_kim_cao_gundogdu_2013, title={Many-Body Effects in Valleytronics: Direct Measurement of Valley Lifetimes in Single-Layer MoS2}, volume={14}, ISSN={1530-6984 1530-6992}, url={http://dx.doi.org/10.1021/nl403742j}, DOI={10.1021/nl403742j}, abstractNote={Single layer MoS2 is an ideal material for the emerging field of "valleytronics" in which charge carrier momentum can be finely controlled by optical excitation. This system is also known to exhibit strong many-body interactions as observed by tightly bound excitons and trions. Here we report direct measurements of valley relaxation dynamics in single layer MoS2, by using ultrafast transient absorption spectroscopy. Our results show that strong Coulomb interactions significantly impact valley population dynamics. Initial excitation by circularly polarized light creates electron-hole pairs within the K-valley. These excitons coherently couple to dark intervalley excitonic states, which facilitate fast electron valley depolarization. Hole valley relaxation is delayed up to about 10 ps due to nondegeneracy of the valence band spin states. Intervalley biexciton formation reveals the hole valley relaxation dynamics. We observe that biexcitons form with more than an order of magnitude larger binding energy compared to conventional semiconductors. These measurements provide significant insight into valley specific processes in 2D semiconductors. Hence they could be used to suggest routes to design semiconducting materials that enable control of valley polarization.}, number={1}, journal={Nano Letters}, publisher={American Chemical Society (ACS)}, author={Mai, Cong and Barrette, Andrew and Yu, Yifei and Semenov, Yuriy G. and Kim, Ki Wook and Cao, Linyou and Gundogdu, Kenan}, year={2013}, month={Dec}, pages={202–206} }
 @article{li_huang_snigdha_yu_cao_2012, title={Role of Boundary Layer Diffusion in Vapor Deposition Growth of Chalcogenide Nanosheets: The Case of GeS}, volume={6}, ISSN={["1936-086X"]}, DOI={10.1021/nn303745e}, abstractNote={We report a synthesis of single-crystalline two-dimensional GeS nanosheets using vapor deposition processes and show that the growth behavior of the nanosheet is substantially different from those of other nanomaterials and thin films grown by vapor depositions. The nanosheet growth is subject to strong influences of the diffusion of source materials through the boundary layer of gas flows. This boundary layer diffusion is found to be the rate-determining step of the growth under typical experimental conditions, evidenced by a substantial dependence of the nanosheet's size on diffusion fluxes. We also find that high-quality GeS nanosheets can grow only in the diffusion-limited regime, as the crystalline quality substantially deteriorates when the rate-determining step is changed away from the boundary layer diffusion. We establish a simple model to analyze the diffusion dynamics in experiments. Our analysis uncovers an intuitive correlation of diffusion flux with the partial pressure of source materials, the flow rate of carrier gas, and the total pressure in the synthetic setup. The observed significant role of boundary layer diffusions in the growth is unique for nanosheets. It may be correlated with the high growth rate of GeS nanosheets, ~3-5 μm/min, which is 1 order of magnitude higher than other nanomaterials (such as nanowires) and thin films. This fundamental understanding of the effect of boundary layer diffusions may generally apply to other chalcogenide nanosheets that can grow rapidly. It can provide useful guidance for the development of general paradigms to control the synthesis of nanosheets.}, number={10}, journal={ACS NANO}, author={Li, Chun and Huang, Liang and Snigdha, Gayatri Pongur and Yu, Yifei and Cao, Linyou}, year={2012}, month={Oct}, pages={8868–8877} }