Session R46: Optical Spectroscopic Measurements of 2D Materials

Internal Photoemission Spectroscopy of 2-D Materials

Recent research has shown the great benefits of using 2-D materials in the tunnel field-effect transistor (TFET), which is considered a promising candidate for the beyond-CMOS technology. The on-state current of TFET can be enhanced by engineering the band alignment of different 2D-2D or 2D-3D heterostructures. Here we present the internal photoemission spectroscopy (IPE) approach to determine the band alignments of various 2-D materials, in particular SnSe$_{2}$ and WSe$_{2}$, which have been proposed for new TFET designs. The metal-oxide-2-D semiconductor test structures are fabricated and characterized by IPE, where the band offsets from the 2-D semiconductor to the oxide conduction band minimum are determined by the threshold of the cube root of IPE yields as a function of photon energy. In particular, we find that SnSe$_{2}$ has a larger electron affinity than most semiconductors and can be combined with other semiconductors to form near broken-gap heterojunctions with low barrier heights which can produce a higher on-state current. The details of data analysis of IPE and the results from Raman spectroscopy and spectroscopic ellipsometry measurements will also be presented and discussed.   

Optical Parameter Extraction of Nano-Layered Materials Using Terahertz Time-Domain Spectroscopy

We report a data analysis technique for reflection-mode terahertz time-domain spectroscopy (THz-TDS) to extract the complex refractive index of nano-layered materials deposited on optically thick substrates. We measure the Fabry-Perot resonances occurring inside the substrate to determine the Fresnel coefficients at the interface of the material and the substrate. Based on these values, we extract the frequency-dependent optical parameters, including surface conductivity, of the nano-layered materials for frequencies up to 3 THz.   

Broadband THz Spectroscopy of 2D Nanoscale Materials

Two-dimensional (2D) materials such as graphene and transition-metal dichalcogenides (TMDC) have attracted intense research interest in the past decade. Their unique electronic and optical properties offer the promise of novel optoelectronic applications in the terahertz regime. Recently, generation and detection of broadband terahertz (~10 THz bandwidth) emission from 10-nm-scale LaAlO$_3$/SrTiO$_3$ nanostructures created by conductive atomic force microscope (c-AFM) lithography has been demonstrated \footnote{Y. Ma, \textit{et al.}, Nano Lett. \textbf{13}, 2884 (2013)}. This unprecedented control of THz emission at 10 nm length scales creates a pathway toward hybrid THz functionality in 2D-material/LaAlO$_3$/SrTiO$_3$ heterostructures. Here we report initial efforts in THz spectroscopy of 2D nanoscale materials with resolution comparable to the dimensions of the nanowire (10 nm). Systems under investigation include graphene, single-layer molybdenum disulfide (MoS$_2$), and tungsten diselenide (WSe$_2$) nanoflakes.   

A novel grating-imaging method to measure carrier diffusion coefficient in graphene

Similar to carrier mobility, carrier diffusion coefficient in graphene determines the response rate of future graphene-based electronics. Here we present a simple, sensitive and non-destructive technique integrated with ultrafast pump-probe spectroscopy to measure carrier diffusion in CVD-grown graphene. In the method, the pump and the probe beams pass through the same area of a photomask with metal strips i.e. a transmission amplitude grating, and get diffracted. The diffracted light is collected by an objective lens and focused onto the sample to generate carrier density grating. Relaxation of this carrier density grating is governed by both carrier recombination and carrier diffusion in the sample. Transient transmission change of the probe beams, which reflects this relaxation process, is recorded. The measured diffusion coefficients of multilayer and monolayer CVD-grown graphene are 2000cm$^{\mathrm{2}}$/s and 10000cm$^{\mathrm{2}}$/s, respectively, comparable with the reported values of epitaxial graphene and reduced graphene. This transmission grating technique can be used to measure carrier dynamics in versatile 2D materials.   

Temperature, Magnetic Field, and Dimensionality Effects on the Raman Spectra of TaSe$_2$

In bulk form, TaSe$_2$ exhibits transitions between commensurate and incommensurate charge-density wave (CDW) phases, and is attracting interest for advanced device applications. In order to explore the evolution of the groundstate CDW phase with layer number, mechanical exfoliation of bulk crystals provides few- to single-layer flakes. In the present work, we extend our opto-thermal Raman measurements\footnote{R. Yan, J. R. Simpson, \textit{et al.}, ACS Nano \textbf{8}, 986 (2014).} on MoS$_2$ to include other TMDs, specifically TaSe$_2$, in both $1T$ and $2H$ crystallographic phases. A novel, magneto-Raman microscope system affords measurement of low-frequency (down to 10\,cm$^{-1}$) vibrational modes as a function of both temperature (100 to 400)\,K and magnetic field (0 to 9)\,T. The dependence of the observed Raman-active phonons on temperature and magnetic field will be discussed and compared with earlier results on MoS$_2$. Specifically, we observe the appearance of low-frequency, zone-folded modes in the CDW state, which soften with temperature similar to the higher frequency, in-plane $E_{2g}$ mode. Additionally, we compare the measured magneto-Raman results to calculations using \textit{ab initio}, density functional theory.   

Ultralow-frequency interlayer Raman modes to probe interfacial coupling in twisted bilayer MoS$_{2}$

Interlayer coupling strength plays an important role in tuning the optoelectronic properties of transition metal dichalcogenides (TMDs), which can be studied in twisted bilayer TMDs due to their various stacking configurations. In this work, ultralow-frequency interlayer shear and breathing Raman modes were investigated in twisted bilayer MoS$_{2}$. We found both twisted angle and translational shift can significantly influence the interlayer coupling, leading to notable frequency and intensity changes of low-frequency Raman modes, as confirmed by first-principles density functional theory calculations. Large frequency and intensity variations occur near twisted angles 0\textordmasculine and 60\textordmasculine , but not between 20\textordmasculine and 40\textordmasculine , indicating translational shift does not induce much change of the coupling strength within the latter angle range. In contrast to low-frequency interlayer modes, high-frequency intralayer Raman modes are much less sensitive to interlayer coupling. Therefore, interlayer Raman modes can be used as an effective probe to study the interlayer coupling of 2D materials with different stacking configurations.   

Photocurrent spectroscopy of 2D materials

Confocal photocurrent measurements provide a powerful means of studying many aspects of the optoelectronic and electrical properties of a 2D device or material. At a diffraction-limited point they can provide a detailed absorption spectrum, and they can probe local symmetry, ultrafast relaxation rates and processes, electron-electron interaction strengths, and transport coefficients. We illustrate this with several examples, once being the photo-Nernst effect. In gapless 2D materials, such as graphene, in a perpendicular magnetic field a photocurrent antisymmetric in the field is generated near to the free edges, with opposite sign at opposite edges. Its origin is the transverse thermoelectric current associated with the laser-induced electron temperature gradient. This effect provides an unambiguous demonstration of the Shockley-Ramo nature of long-range photocurrent generation in gapless materials. It also provides a means of investigating quasiparticle properties. For example, in the case of graphene on hBN, it can be used to probe the Lifshitz transition that occurs due to the minibands formed by the Moire superlattice. We also observe and discuss photocurrent generated in other semimetallic (WTe$_{\mathrm{2}})$ and semiconducting (WSe$_{\mathrm{2}})$ monolayers. Work supported by DoE BES and NSF EFRI grants.   

Two-Dimensional Transition Metal Dichalcogenides: Controlled Synthesis and Optical Characterization

Chemical vapor deposition (CVD) is a bottom-up approach suitable for the synthesis of MoS$_{2}$ and WS$_{2}$ monolayers. In order to extend the application of CVD, we modified the precursors used during the deposition. We show that by using mixed transition metal precursors of MoS$_{2}$/WO$_{3}$ powders, alloyed monolayers of Mo$_{x}$W$_{1-x}$S$_{2}$ islands can be synthesized exhibiting a compositional gradient and a tunable optical band gap, as confirmed by Raman and photoluminescence measurements. We further show that adding tellurium powders into the transition metal precursors can lead to a 200 $^{o}$C reduction in the synthesis temperature for MoS$_{2}$ and WS$_{2}$ monolayers. The materials synthesized at a reduced temperature maintain a high degree of crystallinity and optical properties.   

Accuracy of the 2D sheet model for atomically thin layers

The 2D sheet model provides a useful and concise description of the optics of atomically thin layers. The 2D sheet model is mathematically equivalent to the conventional thin slab model in the limit where the optical thickness of the layer is negligible. In this paper, we present a detailed analysis of the accuracy of the 2D sheet model for atomically thin layers by comparing numerically the predicted optical response for representative monolayer 2D materials to that obtained from the conventional thin slab model. The agreement between the optical responses produced by the two models is found to be within 0.1\%, demonstrating excellent accuracy of the sheet model. Based on the 2D sheet model, we then derive the frequently applied linearized relations between the optical contrasts and the sheet conductivity. The linearized relations provides good accuracy when the material response is weak, but is shown to produce an inaccuracy of more than 25\% in certain wavelength window for even a single atomic layer of MoS$_\text{2}$. With the expression for optical transmission from the sheet model, we will clarify a confusion that occasionally arises when determining the optical attenuation by a thin layer.   

Ultrasensitive Molecular Sensor Using N-doped Graphene through Enhanced Raman Scattering

As a novel and efficient surface analysis technique, graphene enhanced Raman scattering (GERS) has attracted increasing research attention in recent years. In particular, chemically doped graphene demonstrates much enhanced GERS effects than pristine graphene (PG) and it can be used to efficiently detect trace amount of molecules. However, the GERS mechanism is still an open question. Here, we present a comprehensive study on the GERS effect of PG and nitrogen-doped graphene (NG). By controlling the N-doping in NG, the Fermi level of graphene shifts, and if this shift aligns with the lower unoccupied molecular orbital (LUMO) of a molecule, charge transfer is enhanced, thus significantly amplifying the molecule vibrational Raman modes. We confirmed these findings using different organic fluorescent molecules. Interestingly, Raman signals from these dye molecules can be detected even for concentrations as low as 10$^{\mathrm{-11}}$mol/L, thus providing excellent molecular sensing capabilities. In order to explain our results, these NG-molecule systems were modeled using dispersion corrected density functional theory. Furthermore, we demonstrated that when using different laser excitations, it is possible to determine the gaps between the HOMO and LUMO of different molecules.   

Optical/Electronic Heterogeneity of WSe$_{\mathrm{2}}$ at the Nanoscale

Many classes of two-dimensional (2D) materials have emerged as a potential platform for novel electronic and optical devices. However, the physical properties are strongly influenced by nanoscale heterogeneities in the form of nucleation sites, defects, strains, and edges. Here we demonstrate nano-optical imaging of the associated influence on structure and electronic properties with sub-20 nm spatial resolution from combined tip-enhanced Raman scattering (TERS) and photoluminescence (TEPL) spectroscopy and imaging. In monolayer WSe$_{\mathrm{2}}$ micro-crystals grown by physical vapor deposition (PVD), we observe significant variations in TERS and TEPL near crystal edges and atomic-scale grain boundaries (GBs), consistent with variations in strain and/or exciton diffusion. Specifically, theoretical exciton diffusion lengths (25 nm) at GBs and heterogeneous nanoscale (30-80 nm) PL emission including a spectral blue-shift at edges are experimentally probed. Further, we are able to engineer the local bandgap of WSe$_{\mathrm{2}}$ crystals by dynamic AFM-control in reversible (24 meV) and irreversible (48 meV) fashions, enabling systematic in-situ studies of the coupling of mechanical degrees of freedom to the nanoscale electronic properties in layered 2D materials.   

Photovoltaic Response from Multilayered Transition Metal Dichalcogenides p-n Junctions

Transition metal dichalcogenides (TMDs) are layered semiconductors with indirect band gaps comparable to Si. These compounds can be grown in large area, while their gap(s) can be tuned by changing their chemical composition or by applying a gate voltage. The experimental evidence collected so far points toward a strong interaction with light, which contrasts with the small photovoltaic efficiencies $\eta \leq 1\%$ extracted from bulk crystals or exfoliated monolayers. Here, we evaluate the potential of these compounds by studying the photovoltaic response of electrostatically generated p-n junctions composed of approximately 10 atomic layers of MoSe$_2$ stacked onto the dielectric \textit{h}-BN. In addition to ideal diode-like response, we find that these junctions can yield, under AM-1.5 illumination, photovoltaic efficiencies $\eta$ exceeding 14\%, with fill factors of $\sim 70\%$. Given the available strategies for increasing $\eta$ such as gap tuning, improving the quality of the electrical contacts, or the fabrication of tandem cells, our study suggests a remarkable potential for photovoltaic applications based on TMDs.   

Raman 2D response of graphene in hBN sandwich as a function of doping

Graphene on $SiO_2$ is plagued by accidental strain and charge doping which cause significant deterioration in electrical, thermal and optical properties. The stacking of Van der Waals layers can not only provide better properties, e.g., electrical mobility, but can also be used for novel interactions between layers. Here we use gated and contacted hBN-graphene-hBN heterostructures to calibrate the 2D Raman response to doping, particularly the low doping region less than $1\times10^{12}$$cm^{-2}$. This will enable the use of the correlation between Raman G and 2D band to determine effects from doping and strain or compression separately. The dielectric environment of hBN as compared to $SiO_2$ affects the phonon dispersion and the Fermi velocity which results in approximately 7 $cm^{-1}$ blue shift in 2D band per side of graphene contacted with hBN. Charge dependent Raman measurements of the G band provide the means to determine the electron-phonon coupling and the Fermi velocity for graphene in an hBN sandwich.