Band Alignment at Au/MoS 2 Contacts: Thickness Dependence of Exfoliated Flakes

Cho

et al. 2018

Adv Materials Inter

Self Cite

bandgap energy of TMDCs is in the visible range, and thus intensive research efforts have been devoted to realize highefficiency TMDC-based optoelectronic devices. [1][2][3][4][5][6][7] TMDC materials have usually been prepared on SiO 2 /Si wafers by mechanical exfoliation from the bulk or thin film deposition for physical characterization and device fabrication. After TMDC sample preparation, Raman and photoluminescence (PL) spectroscopic measurements are usually performed to identify the thickness and bandgap of the sample. [8][9][10][11][12][13][14][15][16][17][18][19][20][21] The optical absorption and emission signals of the sample become larger as the electric field (E-field) intensity of the incident light in the sample increases. In the SiO 2 /Si substrates, the light undergoes multiple reflections in the SiO 2 layer, leading to interference between its partial waves. Local maxima of the E-field intensity appear when the SiO 2 thickness is equal to (2m − 1)λ/4n SiO2 , where m is a positive integer, λ is the light wavelength, and n SiO2 is the refractive index of SiO 2 , which are identified as Fabry-Perot (FP) resonances. [9][10][11][12][13] At every FP resonance, TMDC layers on SiO 2 /Si substrates can produce very strong Raman and PL intensities. To take advantages of the FP resonance, the dielectric layer is at least a quarter-wave in thickness and most of the researchers have used SiO 2 /Si substrates with ≈300 nm thick SiO 2 layers. [9][10][11][12][13] The light-matter interactions in extremely thin TMDC layers can be strengthened using plasmonic nanoantennae, photonic nanostructures, and the FP cavities. [11,[13][14][15][16][17][18][19][20] Generally, the absorption enhancement is observed only within narrow ranges of the wavelength and angle of the incident light. [9][10][11][12][13][14][15][16][17][18][19][20] To aid material characterization, we can choose an optimal nanostructure to maximize the absorption of a TMDC sample at a specific wavelength. However, such wavelength-and anglesensitive absorption is inappropriate for some applications, including photovoltaics, photodetection, and photocatalysis. [5][6][7] As an alternative approach, Jariwala et al. [7] demonstrated nearunity, broadband absorption in <15 nm thick TMDC layers on Ag and Au back-reflecting substrates. Because TMDC materials usually have a complex refractive index n n ik  = + with an extremely high extinction coefficient (k) in the visible spectrum, The optical characteristics of MoS 2 monolayers on SiO 2 /Si substrates with an SiO 2 thickness ranging from 40 to 130 nm are investigated. The measured Raman and optical reflection spectra of the MoS 2 monolayers vary considerably depending on the SiO 2 thickness. The Raman peak intensity of the MoS 2 monolayer on the substrate with an 80 nm thick SiO 2 layer is four times larger than those in the cases of 40-and 130 nm thick SiO 2 layers, indicating a significant difference in the absorption at the excitation wavelength. The incident light undergoes anomalous phase changes upon r...

Section: Methodsmentioning

confidence: 99%

mentioning

confidence: 99%

Interference‐Enhanced Broadband Absorption of Monolayer MoS₂ on Sub‐100 nm Thick SiO₂/Si Substrates: Reflection and Transmission Phase Changes at Interfaces

Cho

et al. 2018

Adv Materials Inter

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“…Kelvin probe force microscopy (KPFM), a scanning-probe-based technique, has been widely used to measure local surface potential ( V S ) 11–21 . The V S measurements of TMD materials are very useful to identify the number of layers 11–13 , study the photo-carrier generation processes 13,14 and estimate the built-in potential at the heterojunctions 15,16 . Since surface adsorbed molecules, such as H 2 O and O 2 , can transfer charges to the TMD materials, the measurement environment and the sample preparation procedures can significantly vary the measured V S of TMD materiaels 11–14,17,18 .…”

Section: Introductionmentioning

confidence: 99%

“…The light exposure can cause not only the photo-carrier generation/separation but also charge transfer from/to adsorbed/desorbed molecules. All these processes should be considered to explain the light-induced V S change 13,14,19,20 . Careful measurements and analyses of the V S of TMD-metal nanostructures will enable us to achieve better understanding of their physical characteristics.…”

Section: Introductionmentioning

confidence: 99%

Light-Induced Surface Potential Modification in MoS2 Monolayers on Au Nanostripe Arrays

Kwon

Song

et al. 2019

Sci Rep

Self Cite

In this work, the surface potential (VS) of exfoliated MoS2 monolayers on Au nanostripe arrays with period of 500 nm was investigated using Kelvin probe force microscopy. The surface morphology showed that the suspended MoS2 region between neighboring Au stripes underwent tensile-strain. In the dark, the VS of the MoS2 region on the Au stripe (VS-Au) was larger than that of the suspended MoS2 region (VS-S). However, under green light illumination, VS-Au became smaller than VS-S. To explain the VS modification, band diagrams have been constructed taking into consideration not only the local strain but also the electronic interaction at the MoS2/Au interface. The results of this work provide a basis for understanding the electrical properties of MoS2-metal contacts and improving the performance of MoS2-based optoelectronic devices.

“…Depletion region formation at MoS 2 homojunctions, heterojunctions, and metal junctions has been modeled and investigated by many groups [17,18,19,20]. Nipane et al [21] modeled the electrostatics of lateral junctions in atomically thin materials using line charges representation, Sohn et al [22] investigated the electrostatic band alignment at Au/MoS 2 contacts as a function of the thickness of MoS 2 exfoliated flakes, and Chiu et al [23] determined the band alignment in single-layer MoS 2 /WSe 2 heterojunctions using micro X-ray photoelectron spectroscopy and scanning tunneling microscopy (STM).…”

Section: Introductionmentioning

confidence: 99%

Pinch-Off Formation in Monolayer and Multilayers MoS2 Field-Effect Transistors

2019

The discovery of layered materials, including transition metal dichalcogenides (TMD), gives rise to a variety of novel nanoelectronic devices, including fast switching field-effect transistors (FET), assembled heterostructures, flexible electronics, etc. Molybdenum disulfide (MoS2), a transition metal dichalcogenides semiconductor, is considered an auspicious candidate for the post-silicon era due to its outstanding chemical and thermal stability. We present a Kelvin probe force microscopy (KPFM) study of a MoS2 FET device, showing direct evidence for pinch-off formation in the channel by in situ monitoring of the electrostatic potential distribution along the conducting channel of the transistor. In addition, we present a systematic comparison between a monolayer MoS2 FET and a few-layer MoS2 FET regarding gating effects, electric field distribution, depletion region, and pinch-off formation in such devices.