Session L45: Surface Studies of Transition Metal Chalcogenides

Layer and Growth Temperature Dependent Electronic Properties of VSe2 Thin Films

TMDC provide a unique platform exhibiting ground states from CDW, to ferromagnetism and superconductivity. Among the TMDC family, bulk 1-T VSe2 is known as a typical 3D CDW material with a transition temperature of 105 K. But there have been two interesting observations in a few layers VSe2. First, while bulk VSe2 is paramagnetic, VSe2 a few layers limit was shown ferromagnetism. Second, multiple CDW patterns with different Q-vectors have been observed. In this work, we grow VSe2 thin films at two growth temperatures (Tg) by a home-built MBE and study the samples in a 4K STM. In the samples grown at low Tg, we found different 2D CDW patterns in the first and second layer. The high Tg films show stripe-type topographies in the first and second layer with different periodicity. The STM scans and RHEED images indicate that the second layer grown at high Tg is 1T’ phase VSe2. Based on this we conclude that two different Tg produce distinct polymorphs of VSe2. We further compare the dI/dV spectrums of the two polymorphs as a function of layer thickness and discuss the evolution of the electronic properties with thickness.   

Transport properties of high-mobility InSe in Quantum Hall regime.

Among the modern two-dimensional (2D) semiconductors, InSe is a promising candidate for future electronics due to its large thickness-dependent bandgap and high mobility. We have fabricated high mobility ultrathin InSe devices, and observed quantum Hall states in high magnetic field. Transport properties of InSe at low temperatures and high magnetic fields will be presented.   

New Mechanical Exfoliation Technique for Preparing Large Area 2D Materials and Special Structures

Mechanical exfoliation method has been widely used to study the intrinsic properties of 2D materials. Even though the exfoliated 2D materials show high quality, the size of monolayer samples are usually quite small, which has limited the investigation progress of 2D materials. Recently, we developed a new mechanical exfoliation technique for preparing large area and high quality 2D materials^[1]. Many monolayer 2D materials with milimeter to centermeter size have been successfully exfoliated through this method, including graphene, TMDCs et al. The key of this technique is to enhance the van der Waals interaction between the layered materials and the substrates, which can be realized by optimizing the exfoliation process, such as substrate types, temperature and vacuum. Besides, some special structures (like bubble and wrinkle) can be prepared by using different parameters. Therefore, many unique properties can be observed on these structures, e.g., standing wave induced Raman oscillation was first discovered on the exfoliated graphene bubbles^[2]. The new mechanical technique will show great potential for exploring new properties of 2D materials.


References
[1] Y. Huang, et al., ACS nano, 2015, 9, 10612.
[2] Y. Huang, et al., Phys. Rev. Lett., 2018, 120, 186104.   

Distinct ultrafast carrier dynamics governed by spin filtering in bulk and single-layer WSe2

Carrier dynamics in a semiconductor following pulsed optical excitation is relevant to ultrafast optoelectronic applications. We report herein a comparative study of bulk WSe₂ and single-layer (SL) WSe₂ grown on bilayer-graphene-terminated SiC. Subtle dimensional effects lead to substantial differences in the transient response as revealed by time-resolved angle-resolved photoemission spectroscopy (TRARPES) measurements. The conduction band minimum in bulk WSe₂, populated by optical pumping, decays promptly. The corresponding decay for SL WSe₂ is more complex but overall much slower due to spin filtering arising from the lack of space inversion symmetry. Concurrent measurements of the valence bands show longer but different response times arising from lattice excitation, carrier transport, and dissipation. These results illustrate the various time scales and processes at work for carrier and band structure evolution.   

Investigation of charge density wave transitions in a proximally coupled single-layer TiTe2/TiSe2 system

TiSe₂, a prototypical charge density wave (CDW) system with a 1T structure in the bulk, undergoes a (2×2×2) CDW transition at 205 K. By comparison, a single layer of TiSe₂ prepared on a bilayer-graphene-terminated SiC(0001) undergoes a (2×2) CDW transition at 232 K. The bonding of the single layer to the substrate is expected to be of the van der Waals type. Furthermore, the structures are incommensurate, which makes the interfacial bonding even weaker. A reasonable premise is that the single layer is nearly freestanding with minimal influence from the substrate on its (2×2) CDW. TiTe₂, by contrast, shows no CDWs in the bulk or in films grown on the same bilayer-graphene-terminated SiC; the only exception is a (2×2) CDW transition at 92 K in the single layer. The unique behavior of the single layer is an anomaly. To explore the physics further, we have grown a composite system made of a single layer of TiTe₂ on a single layer of TiSe₂ on a bilayer-graphene-terminated SiC. The crystal structures of TiSe₂, TiTe₂, and graphene are all incommensurate. Questions abound: Is there a CDW transition in this composite system? Or, are there actually two transitions? Results of band structure mapping by angle-resolved photoemission spectroscopy will be presented to address these issues.   

Electronic and structural properties of the phase transitions in TaTe2-xSex

Charge density wave (CDW) is a spontaneous modulation of electron density, which is usually associated with periodic lattice distortion with the same wavelength. Such phenomenon is often observed in layered transition metal dichalcogenides (TMD), although its origin is still under debate. 1T-TMD TaTe₂ shows a CDW like phase transition around 170 K where new modulation of 3×3 superstructure forms. Magnetic susceptibility and resistivity show abnormal behavior at the phase transition, which is inconsistent with conventional model or example. Here, we use combination of scanning tunneling microscopy/spectroscopy, low energy electron diffraction, and density functional theory calculations to study the temperature dependence of CDW in undoped and Se doped TaTe₂ single crystals. We have observed a hidden phase with a distinct modulation, which can be stabilized by Se doping, and is less metallic than the 3×3 low temperature phase. Our results suggest a possible explanation to the mystery in transport properties, which provides new insight on the electron-lattice coupling associated with CDW phase transitions.   

Confining electronic states with domain walls in ferroelectric SnTe monolayers

Electronic standing wave patterns at surfaces are usually generated by edges, atomic steps or adatoms that introduce large potential barriers. However, in multi-valley semiconducting 2D materials, quasiparticle confinement and suppressed transmission can be created from mismatched electronic structures through domain walls, with a neglectable built-in bias at the interface. Here, we report the first observation of electronic standing waves near the valence band maximum in a SnTe monolayer that is generated by the reflection from ferroelectric domain walls. Despite of the nearly flat potential barrier at the domain walls, the reflectivity is found to be 68+-9% at 4.7 K, and no sign of transmission was seen. Such a strong confinement of electronic states is due to a large change of valley positions between the neighboring domains arising from a 90 degree rotation of the Brillouin zone. These results show potential for polarization-tuned valleytronics in 2D ferroelectrics.   

Electrical and Optical Characterization of Ultrathin Tellurium Nanostructures Synthesized by Vapor Phase Deposition

Tellurium (Te) is a semiconductor with a slightly indirect bulk bandgap of 0.35 eV. The trigonal crystal structure of Te consists of 1D helical chains of Te atoms stacked together by van der Waals type bonds arranged on 2D hexagonal lattice. Each Te atom is covalently bonded with its two nearest neighbors on the same chain. This unique crystal structure allows a stable 1T-MoS₂-like α-Te, a metastable tetragonal β-Te, and a 2H-MoS₂–like γ-Te structures. The α- and γ-Te phases are expected to have nearly direct band gaps and high carrier motilities suggesting potential applications in electronic and optoelectronic devices. Here we present the vapor phase synthesis and characterization of novel ultrathin Te nanostructures having thickness down to 3.5 nm. Room temperature (RT) electrical measurements exhibit p-type semiconductor with field effect hole mobility of ~350 cm²/V.s. The low-temperature transport properties, RT micro-photoluminescence, and structural properties of the Te nanostructures will also be presented.   

Biexcitons in Monolayer Transition Metal Dichalcogenides Tuned by Magnetic Fields

Time-Integrated Four Wave Mixing measurements were performed on CVD grown MoSe₂ monolayers. The dephasing lifetime was investigated in magnetic fields up to 25T. Additionally, the effects of valley selection on the lifetime was explored through varying polarization schemes. The results were then modeled using time-dependent density function theory revealed the dephasing lifetime is dominated by the inter-valley biexcitons.

  

Controlled and Tunable Light Emission of Heat Treated Black Phosphorus: Towards Tunable Optoelectronic Devices

Layered black phosphorus (BP) has shown exceptional optical properties due to its layer-dependent band gap, which ranges from 0.3 to 1.7 eV for bulk to monolayer, respectively. However, instability of black phosphorus nanosheets can be a major hurdle in controlling this light emission and requires various passivation methods. Moreover, the tunablility of the emitted light can only be controlled by changing the nanosheet thickness, which in turns changes the band gap energy. Here, we show for the first time a well-controlled tunable light emission from heat treated black phosphorous nanosheets. We show tunable wideband photoluminescence (PL) between 590nm to 720nm, with a tunablilty resolution of 5nm. This tunable PL is found to be anisotropic and can last for at least 10 days when nanosheets are properly stored. We attribute the origin of this light emission to the formation of stable black phosphorus oxide on the surface of black phosphorus, as confirmed with i-XPSmeasurements. Our findings unlock doors for optoelectronic applications of layered black phosphorus nanosheets.   

Reversible Thermally Driven Phase Transformation in Ultrathin Ferroelectric In2Se3 Layers

Phase transformation in emerging two dimensional (2D) ferroelectric materials is crucial for their applications, such as in nonvolatile memory devices. We combined STM, STEM, Raman spectroscopy and first-principles calculation to investigate ultrathin layered In₂Se₃ synthesized by chemical vapor deposition (CVD). At room temperature, we observed that ultrathin In₂Se₃, ranging from ~20 layers to monolayer, stabilized at the β phase with a superlattice. Strikingly, at around 210 K the β phase converted to a novel and more stable β^′ phase, which has never been revealed in 2D In₂Se₃ with atomic resolution. The thermally driven kinetics of the reversible β-to-β^′ phase transformation was studied with temperature dependent STEM and Raman spectroscopy, which corroborated with the expected minimum-energy pathways for the transformation obtained from our DFT calculation. The calculated energy difference between the β phase and the β^′ phase is 0.033 eV/In₂Se₃, with an energy barrier of 0.006 eV/In₂Se₃. The DFT calculation further suggested in-plane ferroelectricity in the β^′ phase, and in contrast, both in-plane and out-of-plane ferroelectricity in the β phase.