The diffusion and coarsening of two-dimensional homoepitaxial islands on Cu(100) and Ag(100) surfaces have been studied at room temperature with time-sequenced scanning tunneling microscopy. Quantitative analyses of the dependence of island diffusion coefficient D vs the island side length L, D~L 2a , yield noninteger scaling exponents which are consistent with island coalescence. Moreover, the near absence of island decay shows that the island diffusion occurs via mass transport along the island periphery. [S0031-9007(97)04302-0] PACS numbers: 68.35.Fx, 61.16.Ch, 61.46. + w, 82.65.Dp Diffusion of two-dimensional (2D) epitaxial islands on a substrate is an important subject in surface science and thin-film growth. Information about the elementary atomistic processes involved in thin-film growth may be extracted by simply studying the macroscopic island migration. A number of studies [1][2][3][4][5] have aimed at establishing a general scaling relation, D~L 2a , between the diffusion coefficient D of a large island and the island side length L. Three dominant mechanisms have been proposed. If the island motion is caused by evaporation and condensation events as the island exchanges atoms randomly with the 2D (or 3D) gas (EC mechanism), then a 1. If the evaporation and condensation events are correlated, or equivalently, if the island motion is terracediffusion limited (TD mechanism), then a 2. If the island moves as a result of atom diffusion along the island periphery (PD mechanism), then a 3. These models predicting integer exponents describe, however, highly simplified or limiting cases. In fact, noninteger scaling behaviors or size-dependent scaling exponents have been observed in several Monte Carlo simulations [6-8] and detailed theoretical treatments [5,9].Recently, the diffusion and coarsening of very large 2D islands (containing hundreds to thousands of atoms or vacancies) have been investigated with scanning tunneling microscopy (STM) [3,4,[10][11][12], with quantitative measurements reported for Ag(100) [3,11] and Ag(111) [4,12]. For vacancy island diffusion on Ag(111), a value of a ഠ 2 was found and the TD mechanism was suggested to be dominant [4]. Nevertheless, as reported earlier [13], the evaporation rate of atoms from such island edges is nearly 3 orders of magnitude too small to account for the observed island diffusion coefficient. For adatom islands on Ag(100), a very weak (if any) dependence, a ഠ 1, was found, and the EC mechanism was suggested to be dominant [3]. The near integer exponents in both studies seem to support the simplified TD and EC models, and both experiments downplay possible contributions from the PD mechanism. This is questionable, because periphery diffusion is expected to be a faster atomistic process than evaporation of atoms [14].In this Letter, we present the first known example where the periphery diffusion is, in fact, the main mechanism for island diffusion. We carry out a combined study of island diffusion, island coalescence, and island decay for two model syste...

Two-dimensional materials constitute a promising platform for developing nanoscale devices and systems. Their physical properties can be very different from those of the corresponding three-dimensional materials because of extreme quantum confinement and dimensional reduction. Here we report a study of TiTe2 from the single-layer to the bulk limit. Using angle-resolved photoemission spectroscopy and scanning tunneling microscopy and spectroscopy, we observed the emergence of a (2 × 2) charge density wave order in single-layer TiTe2 with a transition temperature of 92 ± 3 K. Also observed was a pseudogap of about 28 meV at the Fermi level at 4.2 K. Surprisingly, no charge density wave transitions were observed in two-layer and multi-layer TiTe2, despite the quasi-two-dimensional nature of the material in the bulk. The unique charge density wave phenomenon in the single layer raises intriguing questions that challenge the prevailing thinking about the mechanisms of charge density wave formation.

Single layers of transition metal dichalcogenides (TMDCs) are excellent candidates for electronic applications beyond the graphene platform; many of them exhibit novel properties including charge density waves (CDWs) and magnetic ordering. CDWs in these single layers are generally a planar projection of the corresponding bulk CDWs because of the quasi-twodimensional nature of TMDCs; a different CDW symmetry is unexpected. We report herein the successful creation of pristine single-layer VSe2, which shows a (√7 × √3) CDW in contrast to the (4 × 4) CDW for the layers in bulk VSe2. Angle-resolved photoemission spectroscopy (ARPES) from the single layer shows a sizable (√7 × √3) CDW gap of ~100 meV at the zone boundary, a 220 K CDW transition temperature twice the bulk value, and no ferromagnetic exchange splitting as predicated by theory. This robust CDW with an exotic broken symmetry as the ground state is explained via a first-principles analysis. The results illustrate a unique CDW phenomenon in the two-dimensional limit.

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Two-dimensional (2D) topological insulators (TIs) are promising platforms for low-dissipation spintronic devices based on the quantum-spin-Hall (QSH) effect, but experimental realization of such systems with a large band gap suitable for room-temperature applications has proven difficult. Here, we report the successful growth on bilayer graphene of a quasi-freestanding WSe2 single layer with the 1T′ structure that does not exist in the bulk form of WSe2. Using angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy/spectroscopy (STM/STS), we observe a gap of 129 meV in the 1T′ layer and an in-gap edge state located near the layer boundary. The system′s 2D TI characters are confirmed by first-principles calculations. The observed gap diminishes with doping by Rb adsorption, ultimately leading to an insulator–semimetal transition. The discovery of this large-gap 2D TI with a tunable band gap opens up opportunities for developing advanced nanoscale systems and quantum devices.