Hexagonal boron nitride is the only substrate that has so far allowed graphene devices exhibiting micrometer-scale ballistic transport. Can other atomically flat crystals be used as substrates for making quality graphene heterostructures? Here we report on our search for alternative substrates. The devices fabricated by encapsulating graphene with molybdenum or tungsten disulfides and hBN are found to exhibit consistently high carrier mobilities of about 60 000 cm(2) V(-1) s(-1). In contrast, encapsulation with atomically flat layered oxides such as mica, bismuth strontium calcium copper oxide, and vanadium pentoxide results in exceptionally low quality of graphene devices with mobilities of ∼1000 cm(2) V(-1) s(-1). We attribute the difference mainly to self-cleansing that takes place at interfaces between graphene, hBN, and transition metal dichalcogenides. Surface contamination assembles into large pockets allowing the rest of the interface to become atomically clean. The cleansing process does not occur for graphene on atomically flat oxide substrates.

In situ band gap mapping of the V 2 O 5 001 crystal surface revealed a reversible metal-to-insulator transition at 350-400 K, which occurs inhomogeneously across the surface and expands preferentially in the direction of the vanadyl (V O) double rows. Supported by density functional theory and Monte Carlo simulations, the results are rationalized on the basis of the anisotropic growth of vanadyloxygen vacancies and a concomitant oxygen loss driven metal-to-insulator transition at the surface. At elevated temperatures irreversible surface reduction proceeds sequentially as V 2 O 5 001 ! V 6 O 13 001 ! V 2 O 3 0001. DOI: 10.1103/PhysRevLett.99.226103 PACS numbers: 68.47.Gh, 68.35.ÿp, 73.20.ÿr Vanadium oxides represent an important class of materials with high potential in many technological applications based on their diverse temperature-dependent electronic, magnetic, and catalytic properties (e.g., [1][2][3][4] and references therein). There is a large variety of the vanadium oxide phases such as VO (V 2 , rocksalt structure),Depending on the ambient conditions and temperature, phase transformations between these oxides can occur. They may involve the formation of mixed valence phases. In addition, several vanadium oxides undergo metal-to-insulator transitions (MIT), e.g., V 2 O 3 at 150 K, VO 2 at 340 K. These complex structural and electronic transformations may play a crucial role in the behavior of vanadia-based systems.Among the surface structures of vanadium oxides [4], the V 2 O 5 001 surface seems to be the most studied [3,[5][6][7][8][9][10][11][12]. This surface exposes vanadyl (V O) double rows along the [010] direction [ Fig. 1(a)], which were observed also with scanning tunneling microscopy (STM) [10,11] and atomic force microscopy [12]. Surprisingly, the V O termination has been found also for the most stable, (0001) surface of V 2 O 3 [4,13,14], whereas the VO 2 110 surface appears to follow the respective bulk termination [1,4,15,16].Since the thermally induced transformations of vanadium oxides may be initiated at the surface and even be restricted to the surface layers, it is important to study their surface structures in the early stages of transitions when several structures may coexist. In this respect, STM combined with scanning tunneling spectroscopy (STS) can provide direct information on both geometrical and electronic structures of oxide surfaces (see, e.g., [17] ). We here report thermally induced reconstructions observed by STM/STS on a V 2 O 5 001 single crystal surface. Data show that V 2 O 5 , which is not known for the MIT in the bulk, exhibits, however, the MIT at the surface which proceeds through the formation and anisotropic growth of vanadyl-oxygen vacancies and is followed by an irreversible surface reduction. The experimental findings are supported by density functional theory (DFT) and

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The pressure dependence of the resonance frequency of several resonant ultrasound spectroscopy modes in a sample of fused silica has been measured at UCLA in atmospheres of air, helium, and argon near ambient temperature. For both compressional and torsional modes, the radiation resistance is linearly dependent upon pressure and increases with the molecular mass of the surrounding gas. The effects are larger for breathing modes than for torsional modes. They also increase with the molecular mass of the gas. A radiation impedance model is presented which explains some of these data qualitatively and quantitatively.