Session R36: Optical Spectroscopic Measurements of 2D Materials

Rapid optical characterization of epitaxial graphene with confocal laser scanning microscopy

Wafer-scale uniformity of graphene can provide a template for the integration of various 2D materials. Quality of large area graphene has been improved significantly over the last decade, either by epitaxial growth on SiC or chemical vapor deposition on Cu. Achievement of perfect single layer graphene over millimeter scales will largely depend on rapid optical characterization. For example, the multilayer regions can be rapidly assessed with a conventional optical microscope. However, the low optical resolution and the 2.3{\%} absorption of single layer graphene makes it difficult to accurately map out sub-micron scale inhomogeneity in graphene. We demonstrate rapid characterization of epitaxial graphene using a confocal laser scanning microscope (CLSM), which can capture images with 12-bits of brightness and resolve features down to 120 nm, whilst maintaining the speed of an optical microscope. Furthermore, the CLSM captures 3-D information of the sample morphology with height resolution of sub-10 nm. Thus, CLSM is a useful technique for rapid characterization of large areas of graphene and other related 2D materials.   

Development of broadband circularly polarized Terahertz spectroscopy

The development of polarization sensitive detection of THz picosecond pulses is being actively pursued with a variety of methods. However, highly sensitive techniques have only been developed with single frequency sources, such as far-infrared lasers, and only recently have similar techniques been shown to work with pulsed THz radiation. One of the jarring deficiencies is that of broadband circularly polarized THz pulses. This is mostly because circular polarization generation is based on birefringent materials that will only generate the right phase shift for a singular combination of thickness and frequency. We have overcome these deficiencies by using an array of Fresnel rhombs made of the plastic Topas. With its constant refractive index throughout the THz range and negligible absorption, it has allowed us to generate true broadband THz pulses. We use these arrays to study the magneto-optical properties of a 2D electron gas in a quantum well of GaAs-AlGaAs. We show explicitly the differences between cyclotron resonance active and inactive circular conductivities. We project that this technique will be extremely powerful in the study of novel states of matter in both 2D and 3D materials.   

Investigation of the thermal stability of 1T'-MoTe$_{\mathrm{2}}$ multilayers via Raman spectroscopy

The distorted octahedral (1T') form of MoTe$_{\mathrm{2}}$ has garnered much interest in recent years because of its potential applications as a quantum spin hall insulator. Here we study the structural stability of 1T'-MoTe$_{\mathrm{2}}$ multilayers encapsulated by hexagonal boron nitride (hBN) above room temperature by tracking the evolution of its Raman spectrum and cross-checking the results with atomic force microscopy (AFM). Our data indicate the presence of both linear and nonlinear redshifts in peak positions upon heating and, furthermore, suggest the irreversible degradation of the original compound into tellurium nanocrystals at higher temperatures. We discuss the implications of these findings for related optical and transport experiments involving this material and how encapsulation may help extend the lifetime of such devices.   

An integrated and multi-purpose microscope for the characterization of atomically thin optoelectronic devices

Optoelectronic devices based on graphene and other two-dimensional (2D) materials, such as transition metal dichalcogenides (TMDs) are the focus of wide research interest. They can be the key to improving bandwidths in telecommunications, capacity in data storage, new features in consumer electronics, safety devices and medical equipment. The characterization these emerging atomically thin materials and devices strongly relies on a set of measurements involving both optical and electronic instrumentation ranging from scanning photocurrent mapping to Raman and photoluminescence (PL) spectroscopy. In this talk I will present an integrated, multi-purpose instrument dedicated to the optical and electrical characterization of devices based on 2D materials which is able to perform low frequency electrical measurements, scanning photocurrent mapping, Raman, absorption and PL spectroscopy in one single set-up with full control over the polarization and wavelength of light.\\ \\In collaboration with: Adolfo De Sanctis, Gareth Jones, Nicola Townsend, Monica Craciun, Monica Craciun, University of Exeter   

Optical properties of two-dimensional charge density wave materials

Titanium diselenide (TiSe$_{2}$) is a member of the layered transition metal dichalcogenide (TMD) materials. It exhibits unusual chiral charge ordering below 190 K after undergoing an initial phase transition to a commensurate (2 x 2 x 2) charge density wave (CDW) at 200 K which is enhanced further in the monolayer [1]. Recently, the first evidence of chirality in a CDW system was discovered in this material by scanning tunneling microscopy and time-resolved reflectivity experiments, where separate left and right handed charge-ordered domains were found to exist within a single sample [2]. We have prepared single crystals of 1T-TiSe$_{2}$ using iodine vapour transport, and confirmed their quality by x-ray analysis and charge transport measurements. Using a combination of polarised optical spectroscopy techniques in the mid to far infrared (4 to 700 meV photon energy), we have measured an anisotropy relating to the CDW gap. We discuss the results on the basis of chiral domains with different handedness and the nature of the CDW transition.\\[4pt] [1] P. Chen et al. Charge density wave transition in single-layer titanium diselenide. Nature Communications, 6:8943, (2015).\\[0pt] [2] J. Ishioka et al. Chiral charge-density waves. Physical Review Letters, 105(17):176401, (2010).   

Experimental artifacts influencing polarization sensitive magneto-Raman spectroscopy

Since the discovery of graphene, there has been an explosion of research on 2D layered materials such as transition metal dichalcogenides (TMDs). Among several experimental techniques utilized for studying these materials, Raman spectroscopy has proven to be a very powerful tool due to it's sensitivity to layer numbers, interlayer coupling etc. Layered MoS$_2$, member of TMD family, is a typical example with promising applications in nano-optoelectronics. A recent magneto-Raman investigations on MoS$_2$ published by J. Ji $\textit{et al}$ reported an observation of giant magneto-optical effect [PNAS \textbf{113}, 2349 (2016)]. In this work, the intensity of Raman modes exhibited dramatic change in intensities and was attributed to field-induced broken symmetry on Raman scattering cross-section. Due to the ambiguous nature of the interpretation presented in this publication, we performed further Raman studies on MoS$_2$ at high magnetic fields to illustrate the experimental factors overlooked by the previous study. It is highly important to consider the magnetic field-induced rotation of the polarization of the light and its effect on the Raman active phonon modes in anisotropic materials. A detailed report of our magneto-Raman experiments and their outcomes will be presented.   

Enhancement of Raman scattering from monolayer graphene by photonic crystal nanocavities

Monolayer graphene is an atomically thin two-dimensional material that shows strong Raman scattering, while photonic crystal nanocavities with small mode volumes allow for efficient optical coupling at the nanoscale.\footnote{X. Gan, K. F. Mak, Y. Gao, Y. You, F. Hatami, J. Hone, T. F. Heinz, and D. Englund, Nano Lett. 12, 5626 (2012).} Here we demonstrate resonant enhancement of graphene Raman G$^\prime$ band by coupling to photonic crystal cavity modes. Hexagonal-lattice photonic crystal L3 cavities are fabricated from silicon-on-insulator substrates,\footnote{R. Watahiki, T. Shimada, P. Zhao, S. Chiashi, S. Iwamoto, Y. Arakawa, S. Maruyama, and Y. K. Kato, Appl. Phys. Lett. 101, 141124 (2012).}$^,$\footnote{X. Liu, T. Shimada, R. Miura, S. Iwamoto, Y. Arakawa, and Y. K. Kato, Phys. Rev. Applied 3, 014006 (2015).} and monolayer graphene sheets grown by chemical vapor deposition are transferred onto the nanocavities. Excitation wavelength dependence of Raman spectra show that the Raman intensity is enhanced when the G$^\prime$ peak is in resonance with the cavity mode. By performing imaging measurements, we confirm that such an enhancement is only observed at the cavity position.   

Raman spectroscopy of ultra-thin black phosphorus with defects

Thin layers Black phosphorus (P-black) have raised interest for their low-dimensional properties. The material exhibits high mobility, tunable bandgap, and other interesting anisotropic optical and electrical properties. However, thin layers of P-black are unstable in air, which represents a challenge for making experiments. Here we present a Raman study on the influence of a degradation of ultra-thin layers on the Raman properties. Our results reveal the presence of defect-induced Raman modes as well as other signatures in the relative Raman intensity. The multiple expositions to ambient conditions provide insight on the nature of theses modes. The analysis of those modes explains the intensity ratio (A1g over A2g) and the appearance of these modes, which involves among other effects, second-order Raman modes.   

Raman spectroscopy of CrSiTe$_{\mathrm{3}}$ multilayers

Ferromagnetism in two-dimensional (2-D) single crystals is a hypothesis under intense scrutiny by theorists and experimentalists alike seeking to model and realize it, respectively. Derived from an intrinsically ferromagnetic bulk compound composed of weakly coupled layers, 2-D CrSiTe$_{\mathrm{3}}$ is an ideal candidate for such a study. Here we correlate the thicknesses of various CrSiTe$_{\mathrm{3}}$ multilayers to their Raman spectra, accounting for the degradative effects of oxidation by fabricating devices and performing measurements in a chemically inert atmosphere. We then passivate the surfaces via encapsulation with hexagonal boron nitride (hBN) before repeating the experiments in air. Our results explain the absence of predicted phonon modes in the few-layer limit of CrSiTe$_{\mathrm{3}}$ as previously observed in the literature and, more importantly, offer a reliable method of identifying the CrSiTe$_{\mathrm{3}}$ monolayers that may potentially settle the debate over ferromagnetic order in reduced dimensionalities.   

Raman signatures of stacking order in 2D Gallium Selenide

Gallium selenide (GaSe) is a van der Waals layered semiconductor with a 2.1-eV bandgap consisting of post-transition metal gallium (Ga) and chalcogen selenium (Se). GaSe can be used in photodevices since GaSe has high photoresponsivity and external quantum efficiency. For example, GaSe-nanosheet-based photodetector has current responsivity of 2.8 A/W while single layer MoS2-based one has only 7.5×10-3 A/W. [1] We investigated more than 40 different GaSe samples by polarized Raman spectroscopy from monolayer to bulk. The A$_{1g}^{1}$ mode blueshifts with increasing layer number while other peaks do not change much. We also observed low frequency shear modes coming from inter-layer vibrations in the range of 5-25 cm$^{-1}$. We found that there are different types of low-frequency Raman spectra even in the same-thickness flakes, which means that GaSe exists with different stacking order domains. In 3-layer GaSe, we found four-types of low-frequency Raman spectra which correspond to different stacking orders. [2] [1] PingAn Hu et al., ACS nano, 6, 5988 (2012) [2] Jae-Ung Lee et al., ACS nano, 10, 1948 (2016)   

Probing excitonic emission in 2D semiconductors with plasmon-assisted spectroscopy

Surface plasmon polaritons (SPPs) exhibit electromagnetic fields that are strongly polarized perpendicular to the metal surfaces on which they propagate. As a result, coupling of dipole emission into SPPs can be utilized as a direct probe of dipole orientation. Furthermore, coupling into SPPs can significantly enhance emission rates due to the reduced SPP mode volume. In this work, we couple transition metal dichalcogenide monolayers, which are two dimensional semiconductors, to silver films by mechanical stamping, and study the relative emission of excitonic states into free space versus SPPs. In addition to regular excitons that have been observed previously, we also observe additional emissive states that exhibit highly selectively coupling to SPPs. We will discuss the origin of these states.   

Electric field induced variations of excited state lifetimes and photoluminescence spectra in 2D heterostructures

Optical properties of 2D nanomaterials are of fundamental interest because of their potential use in integrated optoelectronic devices. To understand fundamental limitations of device operation further insight into the dynamics of photo-excited carriers is needed. Here we present experimental results measured on WS$_{\mathrm{2}}$/MoS$_{\mathrm{2\thinspace }}$vertical heterostructures integrated with electrical contacts on an optically transparent sample stack. Using high resolution, inverted optical immersion spectroscopy, we measure the change in the photoluminescence spectrum and fluorescence decay under vertical electrical fields. We find that the integrated photoluminescence intensity and fluorescence lifetimes can be significantly altered upon application of external electric fields. We quantify the measured effects and discuss the mechanisms responsible for the electric-field induced photoluminescence modifications   

Design and performance of graphene quantum dot bolometric detectors.

We recently demonstrated that quantum dots of epitaxial graphene on SiC are extremely sensitive bolometric detectors in the THz frequency range, with responsivity above 10$^{\mathrm{10}}$ V/W at 2.5 K and NEP for an absorbed THz power about 10$^{\mathrm{-16}}$ W Hz$^{\mathrm{-1}}$/2[1]. Here we investigate how the bolometer response depends on the device design, by varying the graphene area and the number of quantum dots connected in parallel. We also investigate the effect of the gate voltage on the temperature dependence of the quantum dot resistance and its responsivity. Finally, we characterize the frequency dependence of the response and show that the quantum dots are broadband detectors, from THz to UV light. [1] A. El Fatimy, R.L.Myers-Ward, A.K. Boyd, K.M. Daniels, D. K. Gaskill, and P. Barbara, Nature Nanotechnology 11, 335--338 (2016) doi:10.1038/nnano.2015.303.-/.