Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session D2: Focus Session: Beyond Graphene - Optics in 2D Semiconductors I |
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Sponsoring Units: DMP Chair: Farhan Rana, Cornell University Room: 001B |
Monday, March 2, 2015 2:30PM - 2:42PM |
D2.00001: Dependence of Monolayer WS$_{2}$-Substrate Interaction on Substrate Type and Bonding Investigated by High-temperature Raman and Photoluminescence Liqin Su, Yifei Yu, Linyou Cao, Yong Zhang We report the temperature and excitation wavelength dependence of the electronic and vibration properties of epitaxially grown WS$_{2}$ monolayers on different substrates, SiO$_{2}$ and sapphire, using photoluminescence (PL) and Raman spectroscopy with temperatures up to 500 $^{\circ}$C. Similar to our previous study on MoS$_{2}$ (Su \textit{et al}., Nanoscale 6, 4920, 2014), the WS$_{2}$ monolayers are shown to also exhibit strong interaction with substrates, manifesting as that their electronic and optical properties depend sensitively on the substrate type and film-substrate bonding. Raman frequency shifts for E$_{\mathrm{2g}}^{1}(\Gamma)$ and A$_{\mathrm{1g}}(\Gamma)$ modes and PL energy shifts are measured from room temperature up to 500 $^{\circ}$C. Raman spectra shows strong substrate dependence, and the thermal quenching of the PL intensity in the high temperature region reveal nonradiative channels with large activation energies in the order of 0.5 eV. This study suggests the critical need to assess the potential impact of the substrate on the intrinsic properties of such 2-D materials and the opportunities for tailoring their properties by selecting different substrates. [Preview Abstract] |
Monday, March 2, 2015 2:42PM - 2:54PM |
D2.00002: Single Quantum Emitters in Monolayer Tungsten Diselenide Genevieve Clark, John Schaibley, Yu-Ming He, Yu He, M. C. Chen, Y. J. Wei, X. Ding, Qiang Zhang, Jian-Wei Pan, Wang Yao, Chaoyang Lu, Xiaodong Xu Single quantum emitters (SQE's) are central to emerging photonic quantum technologies. While they have been realized in a variety of solid state systems, all solid-state quantum emitters to date are embedded in a three dimensional bulk matrix. We present a new type of single quantum emitter in a two-dimensional system, in the form of neutral excitons localized to defects within atomically thin tungsten diselenide monolayers. These localized excitons show strong photoluminescence with 130 ueV emission lines from two non-degenerate, cross-polarized transitions. Their narrow line width is characteristic of localized exciton emission, and is several orders of magnitude narrower than seen from excitons delocalized in a monolayer. Second-order correlation measurements show strong photon anti-bunching, establishing that these localized excitons are single photon emitters. Magneto-optical measurements reveal an exciton g-factor of 8.7, significantly larger than that of delocalized excitons. SQE's in monolayer WSe$_{\mathrm{2}}$ may offer practical advantages such as efficient photon extraction and scalability, and in-situ control of local environment. [Preview Abstract] |
Monday, March 2, 2015 2:54PM - 3:06PM |
D2.00003: Quantum dots in graphene-like materials Thakshila Herath, Vadym Apalkov We study numerically the electron states in silicene and germanene quantum dots within the effective low energy model of silicene and germanene. The quantum dots are realize through spatial variation of perpendicular electric field, i.e., bias voltage. The energy spectra of such quantum dots are obtained for different parameters of the dots, which are the size of the dot and the strength of external electric field. For cylindrically symmetric spatial profile of electric field, the electron states of the dot are characterized by z-component of the angular momentum. Due to strong spin-orbit interactions in such buckled graphene-like materials, the states in the quantum dots have unique spin texture, which is more pronounced for germanene quantum dots. The dependence of spin polarization of electron states in the quantum dots on the strength of electric field is also obtained. [Preview Abstract] |
Monday, March 2, 2015 3:06PM - 3:42PM |
D2.00004: Ultrafast laser spectroscopy of two-dimensional materials and their heterostructures Invited Speaker: Hui Zhao Monolayer transition metal dichalcogenides are new two-dimensional materials beyond graphene. Recently, extensive studies have revealed several unique properties of these materials and their potential applications in electronic and renewable-energy technologies. Furthermore, it is possible to use these atomic layers as building blocks to fabricate new van der Waals heterostructures with emergent properties. In this talk, I will report our recent ultrafast laser studies of several types of two-dimensional transition metal dichalcogenides and their heterostructures. First, we studied several nonlinear optical processes, such as second harmonic generation, which allows detection of the crystal orientation and symmetry of MoS$_2$ monolayers, and two-photon absorption, which was used to measure the bandgap and exciton binding energy of WSe$_2$ monolayers. Second, we used a transient absorption microscopy technique with high spatial resolution to study exciton dynamics in these materials, and measured their exciton lifetime, diffusion coefficient, and ballistic transport. Third, by performing transient absorption measurements with polarization resolution, we studied spin and valley dynamics of excitons in monolayer MoSe$_2$ and deduced a spin relaxation time of about 9 ps at room temperature. Finally, we used the transient absorption technique with layer selectivity to study heterostructures of graphene-WS$_2$, MoS$_2$-MoSe$_2$, and WS$_2$-MoSe$_2$. We observed ultrafast and efficient charge and exciton transfer across the van der Waals interface in all these structures. The formation of spatially indirect excitons in the transition-metal-dichalcogenide heterostructures was also studied. Furthermore, we found that the optical properties of WS$_2$ can be effectively tuned by carriers in graphene in the graphene-WS$_2$ heterostructure. [Preview Abstract] |
Monday, March 2, 2015 3:42PM - 3:54PM |
D2.00005: Pressure-Dependent Optical and Vibrational Properties of Monolayer Molybdenum Disulfide Avinash Nayak, Tribhuwan Pandey, Damien Voiry, Jin Liu, Samuel Moran, Ankit Sharma, Cheng Tan, Chang-Hsiao Chen, Lain-Jong Li, Manish Chhowalla, Jung-Fu Lin, Abhishek Singh, Deji Akinwande Controlling the band gap by tuning the lattice structure through pressure engineering is a relatively new route for tailoring the optoelectronic properties of two dimensional (2D) materials. Here we investigate the electronic and lattice vibrational dynamics of the distorted monolayer 1T-MoS$_{\mathrm{2}}$ (1T') and the monolayer 2H-MoS$_{\mathrm{2}}$ \textit{via} a diamond anvil cell (DAC) and density functional theory (DFT) calculations. The direct optical band gap of the monolayer 2H-MoS$_{\mathrm{2}}$ increases by 11.7{\%} from 1.85 eV to 2.08 eV, which is the highest reported for a 2D transition metal dichalcogenide (TMD) material. [Preview Abstract] |
Monday, March 2, 2015 3:54PM - 4:06PM |
D2.00006: Optoelectronic Crystal of Artificial Atoms in Strain-Textured MoS$_{2}$ Alex W. Contryman, Hong Li, Alex H. Fragapane, Xiaofeng Qian, Sina Moeini Ardakani, Yongji Gong, Xingli Wang, Jeffrey M. Weisse, Chi Hwan Lee, Jiheng Zhao, Pulickel M. Ajayan, Ju Li, Xiaolin Zheng, Hari C. Manoharan The atomically thin semiconductor MoS$_2$ possesses exceptional strength and a strain-tunable band gap. When subjected to biaxial elastic strain, monolayer MoS$_2$ can embed wide band gap variations overlapping the visible spectrum, with calculations showing the modified electronic potential emanating from point-induced tensile strain perturbations mimic the Coulomb potential in a mesoscopic atom. We have realized and confirmed this ``artificial atom'' concept via capillary-pressure-induced nanoindentation of monolayer MoS$_2$ from a tailored nanostructure. We demonstrate that a synthetic lattice of these building blocks forms an optoelectronic crystal capable of broadband light absorption and efficient funneling of photogenerated excitons to points of maximum strain at the atom centers. Such 2D semiconductors with spatially textured band gaps represent a new class of materials which may find applications in next-generation optoelectronics or photovoltaics. [Preview Abstract] |
Monday, March 2, 2015 4:06PM - 4:18PM |
D2.00007: Electronic and optical properties of single-layer, double-layer, and bulk SnSe and GeSe Emmanouil Kioupakis, Guangsha Shi We used density functional and many-body perturbation theory to calculate the quasiparticle band structures and optical properties of single-layer, double-layer, and bulk SnSe and GeSe. The calculated direct and indirect band gaps of the bulk materials are in good agreement with experiment. While the electronic band gaps increase by up to 600 meV in the single-layer, double-layer, and bulk SnSe, the transition energy of the n = 1 exciton does not change as a function of thickness. The same trend was also discovered in GeSe. The fundamental band gaps were found to be direct in SnSe and GeSe monolayers. We calculated the absorption spectra for both the bulk and 2-dimensional systems, and determined the light absorbance for light polarization along the in-plane armchair and zigzag directions. This research was supported by the National Science Foundation CAREER award through Grant No. DMR-1254314. Computational resources were provided by the DOE NERSC facility. [Preview Abstract] |
Monday, March 2, 2015 4:18PM - 4:30PM |
D2.00008: Light-matter interactions of monolayer semiconductors integrated with photonic microcavities Y.-J. Chen, T. Stanev, G. Wei, N. P. Stern, J. D. Cain, V. Dravid Enhanced light-matter interactions in optical microcavities can enable hybrid photon-exciton quasiparticle excitations when in a regime of strong light-matter coupling. Because of their direct bandgap, atomic-scale thickness, and strong spin-orbit coupling, monolayers of transition metal dichalcogenides (TMDs) allow for exciton-polaritons in a two-dimensional regime with rich correlations between spin, momentum, and light polarization. We demonstrate integrated TMD photonic devices with MoS$_2$ grown by vapor transport and sandwiched between dielectric Bragg mirrors. We discuss evidence for exciton-polaritons in monolayer TMDs at room temperature using angle-resolved cavity reflectivity spectroscopy. This interpretation is supported by the dependence on MoS$_2$ layer number. Calculations of light-matter coupling parameters in TMDs yield values consistent with recent observations~\footnote{X. Liu, \textit{et al}. \textit{arXiv}:1406.4826, (2014)}. We discuss our approach to integrated 2D monolayer photonics in the context of the valley-sensitive bandstructure of excitons in TMDs. [Preview Abstract] |
Monday, March 2, 2015 4:30PM - 4:42PM |
D2.00009: Optical Two Dimensional Fourier Transform Spectroscopy of Layered Metal Dichalcogenides P. Dey, J. Paul, C.E. Stevens, Z.D. Kovalyuk, Z.R. Kudrynskyi, A.H. Romero, A. Cantarero, D.J. Hilton, J. Shan, D. Karaiskaj Nonlinear two-dimensional Fourier transform (2DFT) measurements were used to study the mechanism of excitonic dephasing and probe the electronic structure of the excitonic ground state in layered metal dichalcogenides. Temperature-dependent 2DFT measurements were performed to probe exciton-phonon interactions. Excitation density dependent 2DFT measurements reveal exciton-exciton and exciton-carrier scattering, and the lower limit for the homogeneous linewidth of excitons on positively and negatively doped samples. [Preview Abstract] |
Monday, March 2, 2015 4:42PM - 4:54PM |
D2.00010: Layer-structured hexagonal boron nitride carbon semiconductor alloys for deep UV photonics Md Rakib Uddin, Jing Li, Jingyu Lin, Hongxing Jiang Hexagonal boron nitride carbon alloys, $h-$(BN)$_{\mathrm{1-x}}$(C$_{2}$)$_{x}$, are layer-structured semiconductor materials with a tunable bandgap energy from 0 eV (graphite) to 6.5 eV ($h$-BN). We report on synthesizing (BN)-rich $h-$(BN)$_{\mathrm{1-x}}$(C$_{2}$)$_{x}$ semiconductor alloys using standard MOCVD growth technique on sapphire substrate. Bandgap energy variation with carbon concentration in the deep UV spectral range has been demonstrated through optical absorption measurements. Experimental results suggest that the critical carbon concentration ($x_{\mathrm{c}})$ to form the homogenous $h-$(BN)$_{\mathrm{1-x}}$(C$_{2}$)$_{x}$ alloys is about 3.2{\%} at a growth temperature of 1300 $^{\circ}$C. It is expected that homogenous $h-$(BN)$_{\mathrm{1-x}}$(C$_{2}$)$_{x}$ alloys with higher $x$ can be achieved by increasing the growth temperature. This is a huge advantage over the InGaN alloy system in which higher growth temperatures cannot be utilized to close the miscibility gap. Together with our ability for producing high quality $h$-BN epilayers, $h$-(BN)C alloys and quantum wells open up new possibilities for realizing novel 2D optoelectronic devices with tunable physical properties. [Preview Abstract] |
Monday, March 2, 2015 4:54PM - 5:06PM |
D2.00011: Electrically controlled fluorescence quenching of quantum dots on monolayer Molybdenum Disulfide -- Part II Andrey Klots, Dhiraj Prasai, A.K.M. Newaz, Scott Niezgoda, Noah Orfield, Sandra Rosenthal, Kane Jennings, Kirill Bolotin In the second part of this talk, we investigate the mechanisms that enable energy exchange between semiconductor quantum dots (QDs) and two-dimensional (2D) materials. First, we study possible contributions due to multiple mechanisms such as charge transfer, metallic screening, mechanical strain, and Forster resonant energy transfer (FRET). By implementing different 2D materials (graphene, MoS$_{2}$, hexagonal boron nitride), varying their thickness and QD emission wavelengths we demonstrate that QD fluorescence quenching is dominated by FRET. Next, we study the dependence of the FRET rate on electrostatic doping of 2D materials, focusing on the case of monolayer MoS$_{2}$. We develop a simple model, which shows that that moderate (\textless 10{\%}) changes in MoS$_{2}$ absorption induced by gating lead to much larger ($\sim$ 50{\%}) modulation of QD photoluminescence intensity. Finally, we demonstrate that FRET can be used as an efficient spectroscopic tool that probes states in 2D materials that are not accessible via conventional absorption spectroscopy. [Preview Abstract] |
Monday, March 2, 2015 5:06PM - 5:18PM |
D2.00012: Electrically controlled fluorescence quenching of quantum dots on monolayer Molybdenum Disulfide -- Part I Dhiraj Prasai, Andrey Klots, A.K.M. Newaz, Scott Niezgoda, Noah Orfield, Sandra Rosenthal, Kane Jennings, Kirill Bolotin We study hybrid electronic structures in which zero-dimensional semiconductor quantum dots (QDs) are coupled with two-dimensional monolayer molybdenum disulfide (MoS$_{2})$. To fabricate such devices, we mechanically transfer MoS$_{2}$ onto a sub-monolayer of QDs assembled on a functionalized glass surface. We investigate quenching of the fluorescence of QDs which are selectively synthesized to have emission spectra which overlaps with the excitonic absorption peak (2.1eV) in MoS$_{2}$. Both photoluminescence intensity and lifetime for QDs on MoS$_{2}$ decrease $\sim$ 5 times due to near-field energy transfer from QDs to MoS$_{2}$. Furthermore, by electrostatically gating MoS$_{2}$, we control the rate of energy transfer and modulate the photoluminescence intensity of QDs by $\sim$ 50{\%}. [Preview Abstract] |
Monday, March 2, 2015 5:18PM - 5:30PM |
D2.00013: Energy transfer between quantum dots and 2D materials: graphene versus MoS$_2$ Archana Raja, Johanna Zultak, Xiaoxiao Zhang, Andres Montoya-Castillo, Ziliang Ye, Cyrielle Roquelet, Arend van der Zande, Daniel Chenet, Louis Brus, Tony Heinz Understanding charge and energy transfer processes at the interface of nanostructures is an important area of research, both from the fundamental and application points of view. Interactions between 0D semiconductor quantum dots and 2D van der Waals materials have been a subject of recent investigations [1,2]. Here, we report highly efficient near-field energy transfer from core-shell quantum dots to monolayer and few layer graphene, a semi-metal and MoS$_{2}$, a semiconductor. We observe both quenching of single quantum dot photoluminescence (PL) and decreasing lifetime in time resolved PL. Our measurements show that increasing the number of layers in the acceptor van der Waals material results in contrasting trends in the rate of non-radiative energy transfer. The energy-transfer rate increases significantly with increasing layer thickness for graphene, but decreases with increasing thickness for MoS$_{2}$ layers. Energy transfer rates on the order of 1-10ns$^{-1}$ are determined. We interpret the results in terms of differences in the interplay between dielectric loss and screening. \newline [1] Z. Chen, S. Berciaud, C. Nuckolls, T. F. Heinz, and L. E. Brus, ACS Nano 4, 2964 (2010). [2] F. Prins, A. J. Goodman, and W. A. Tisdale, Nano Lett. ASAP, (2014). [Preview Abstract] |
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