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Interfaces impede heat flow in micro/nanostructured systems. Conventional theories for interfacial thermal transport were derived based on bulk phonon properties of the materials making up the interface without explicitly considering the atomistic interfacial details, which are found critical to correctly describing thermal boundary conductance (TBC). Recent theoretical studies predicted the existence of localized phonon modes at the interface which can play an important role in understanding interfacial thermal transport. However, experimental validation is still lacking. Through a combination of Raman spectroscopy and high-energy resolution electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope, we report the first experimental observation of localized interfacial phonon modes at ~12 THz at a high-quality epitaxial Si-Ge interface. These modes are further confirmed using molecular dynamics simulations with a high-fidelity neural network interatomic potential, which also yield TBC agreeing well with that measured from time-domain thermoreflectance (TDTR) experiments. Simulations find that the interfacial phonon modes have obvious contribution to the total TBC. Our findings may significantly contribute to the understanding of interfacial thermal transport physics and have impact on engineering TBC at interfaces in applications such as electronics thermal management and thermoelectric energy conversion.

Cryogen-free dilution refrigerators generally simplify low temperature research but some types of samples, including superconducting qubits and other nanoelectronic devices, are affected by environmental heat sources such as stray photons or residual helium. We present the design and performance of a hermetic cell installed on the mixing chamber plate of a cryogen-free dilution refrigerator. The performance was quantified by measuring the dependence of the resonance frequency of a mechanical oscillator installed inside the cell on the mixing chamber temperature down to 10 mK. We found the expected logarithmic temperature dependence of the resonance frequency down to the lowest temperatures, demonstrating that the efficiency of the hermetic shield is significantly better than that of a simpler shield with no visible gaps.

Silicon double paddle osillators (DPO) have been successfully used for measuring the elastic properties of amorphous films down to 10 mK (see for example \cite{Fefferman16,Liu14}). Until now, our group has used a wet dilution refrigerator for the lowest temperature measurements. We present measurements carried out on a Bluefors cryogen-free dilution refrigerator that demonstrate an extreme sensitivity of the thermal coupling of the DPO to its environment. These measurements show that it is necessary to enclose the DPO in a shield at the mixing chamber (MXC) temperature. Any gaps in the shield limit its effectiveness, even if there is no line-of-sight path to the DPO. In the absence of a cryogenic hermetic shield surrounding the DPO, turning off the pulse tube while maintaining the MXC and still temperatures leads to heating of the DPO. This demonstrates that any heating of the sample due to pulse tube vibrations is a less important effect.

We report on preparation dependent properties observed in monolayer WS2 samples synthesized via chemical vapor deposition (CVD) on a variety of common substrates (Si/SiO2, sapphire, fused silica) as well as samples that were transferred from the growth substrate onto a new substrate. The as-grown CVD materials (as-WS2) exhibit distinctly different optical properties than transferred WS2 (x-WS2). In the case of CVD growth on Si/SiO2, following transfer to fresh Si/SiO2 there is a ~50 meV shift of the ground state exciton to higher emission energy in both photoluminescence emission and optical reflection. This shift is indicative of a reduction in tensile strain by ~0.25%. Additionally, the excitonic state in x-WS2 is easily modulated between neutral and charged exciton by exposure to moderate laser power, while such optical control is absent in as-WS2 for all growth substrates investigated. Finally, we observe dramatically different laser power-dependent behavior for as-grown and transferred WS2. These results demonstrate a strong sensitivity to sample preparation that is important for both a fundamental understanding of these novel materials as well as reliable reproduction of device properties., Comment: final manuscript available at Scientific Reports

The low temperature properties of glass are distinct from those of crystals due to the presence of poorly understood low-energy excitations. The tunneling model proposes that these are atoms tunneling between nearby equilibria, forming tunneling two level systems (TLSs). This model is rather successful, but it does not explain the remarkably universal value of the mechanical dissipation $Q^{-1}$ near 1 kelvin. The only known exceptions to this universality are the $Q^{-1}$ of certain thin films of amorphous silicon, carbon and germanium. Recently, it was found that $Q^{-1}$ of amorphous silicon (a-Si) films can be reduced by two orders of magnitude by increasing the temperature of the substrate during deposition. According to the tunneling model, the reduction in $Q^{-1}$ at 1 kelvin implies a reduction in $P_{0}\gamma ^{2}$, where $P_{0}$ is the density of TLSs and $\gamma $ is their coupling to phonons. In this preliminary report, we demonstrate elastic measurements of a-Si films down to 20 mK. This will allow us, in future work, to determine whether $P_{0}$ or $\gamma $ is responsible for the reduction in $Q^{-1}$ with deposition temperature., Comment: accepted by Journal of Low Temperature Physics

Monolayer WS2 offers great promise for use in optical devices due to its direct bandgap and high photoluminescence intensity. While fundamental investigations can be performed on exfoliated material, large-area and high quality materials are essential for implementation of technological applications. In this work, we synthesize monolayer WS2 under various controlled conditions and characterize the films using photoluminescence, Raman and x-ray photoelectron spectroscopies. We demonstrate that the introduction of hydrogen to the argon carrier gas dramatically improves the optical quality and increases the growth area of WS2, resulting in films exhibiting mm2 coverage. The addition of hydrogen more effectively reduces the WO3 precursor and protects against oxidative etching of the synthesized monolayers. The stoichiometric WS2 monolayers synthesized using Ar+H2 carrier gas exhibit superior optical characteristics, with photoluminescence emission full width half maximum values below 40 meV and emission intensities nearly an order of magnitude higher than films synthesized in a pure Ar environment., Comment: recently accepted at Scientific Reports

Epitaxial graphene layers were grown on the C-face of 4H- and 6H-SiC using an argon-mediated growth process. Variations in growth temperature and pressure were found to dramatically affect the morphological properties of the layers. The presence of argon during growth slowed the rate of graphene formation on the C-face and led to the observation of islanding. The similarity in the morphology of the islands and continuous films indicated that island nucleation and coalescence is the growth mechanism for C-face graphene., Comment: 12 pages, 4 figures

In order to make graphene technologically viable, the transfer of graphene films to substrates appropriate for specific applications is required. We demonstrate the dry transfer of epitaxial graphene (EG) from the C-face of 4H-SiC onto SiO2, GaN and Al2O3 substrates using a thermal release tape. We further report on the impact of this process on the electrical properties of the EG films. This process enables EG films to be used in flexible electronic devices or as optically transparent contacts., Comment: 8 pages, 4 figures and supplementary info regarding procedure for transfer

Graphene was epitaxially grown on both the C- and Si-faces of 4H- and 6H-SiC(0001) under an argon atmosphere and under high vacuum conditions. Following growth, samples were imaged with Nomarski interference contrast and atomic force microscopies and it was found that growth under argon led to improved morphologies on the C-face films but the Si-face films were not significantly affected. Free carrier transport studies were conducted through Hall effect measurements, and carrier mobilities were found to increase and sheet carrier densities were found to decrease for those films grown under argon as compared to high vacuum conditions. The improved mobilities and concurrent decreases in sheet carrier densities suggest a decrease in scattering in the films grown under argon., Comment: 215th Meeting of the Electrochemical Society (ECS 215), 14 pages, 6 figures

Epitaxial graphene films were grown in vacuo by silicon sublimation from the (0001) and (000-1) faces of 4H- and 6H-SiC. Hall effect mobilities and sheet carrier densities of the films were measured at 300 K and 77 K and the data depended on the growth face. About 40% of the samples exhibited holes as the dominant carrier, independent of face. Generally, mobilities increased with decreasing carrier density, independent of carrier type and substrate polytype. The contributions of scattering mechanisms to the conductivities of the films are discussed. The results suggest that for near-intrinsic carrier densities at 300 K epitaxial graphene mobilities will be ~150,000 cm2V-1s-1 on the (000-1) face and ~5,800 cm2V-1s-1 on the (0001) face., Comment: Accepted for publication in Applied Physics Letters, 10 pages, 2 figures

An in vacuo thermal desorption process has been accomplished to form epitaxial graphene (EG) on 4H- and 6H-SiC substrates using a commercial chemical vapor deposition reactor. Correlation of growth conditions and the morphology and electrical properties of EG are described. Raman spectra of EG on Si-face samples were dominated by monolayer thickness. This approach was used to grow EG on 50 mm SiC wafers that were subsequently fabricated into field effect transistors with fmax of 14 GHz., Comment: 215th Meeting of the Electrochemical Society, 8 pages, 8 figures

Graphene layers were created on both C and Si faces of semi-insulating, on-axis, 4H- and 6H-SiC substrates. The process was performed under high vacuum (<10-4 mbar) in a commercial chemical vapor deposition SiC reactor. A method for H2 etching the on-axis sub-strates was developed to produce surface steps with heights of 0.5 nm on the Si-face and 1.0 to 1.5 nm on the C-face for each polytype. A process was developed to form graphene on the substrates immediately after H2 etching and Raman spectroscopy of these samples confirmed the formation of graphene. The morphology of the graphene is described. For both faces, the underlying substrate morphology was significantly modified during graphene formation; sur-face steps were up to 15 nm high and the uniform step morphology was sometimes lost. Mo-bilities and sheet carrier concentrations derived from Hall Effect measurements on large area (16 mm square) and small area (2 and 10 um square) samples are presented and shown to compare favorably to recent reports., Comment: European Conference on Silicon Carbide and Related Materials 2008 (ECSCRM '08), 4 pages, 4 figures

We report a direct correlation between carrier mobility and Raman topography of epitaxial graphene (EG) grown on silicon carbide (SiC). We show the Hall mobility of material on the Si-face of SiC [SiC(0001)] is not only highly dependent on thickness uniformity but also on monolayer strain uniformity. Only when both thickness and strain are uniform over a significant fraction (> 40%) of the device active area does the mobility exceed 1000 cm2/V-s. Additionally, we achieve high mobility epitaxial graphene (18,100 cm2/V-s at room temperature) on the C-face of SiC [SiC(000-1)] and show that carrier mobility depends strongly on the graphene layer stacking. These findings provide a means to rapidly estimate carrier mobility and provide a guide to achieve very high mobility in epitaxial graphene. Our results suggest that ultra-high mobilities (>50,000 cm2/V-s) are achievable via the controlled formation of uniform, rotationally faulted epitaxial graphene., Comment: 13 pages including supplimental material. Submitted to Nature Materials 2/23/2009