We present a comparative study of DNA nucleobases [guanine (G), adenine (A), thymine (T), and cytosine (C)] adsorbed on hexagonal boron nitride (h-BN) sheet and graphene, using local, semilocal, and van der Waals (vdW) energy-corrected density-functional theory (DFT) calculations. Intriguingly, despite the very different electronic properties of BN sheet and graphene, we find rather similar binding energies for the various nucleobase molecules when adsorbed on the two types of sheets. The calculated binding energies of the four nucleobases using the local, semilocal, and DFT+vdW schemes are in the range of 0.54 ∼ 0.75 eV, 0.06 ∼ 0.15 eV, and 0.93 ∼ 1.18 eV, respectively. In particular, the DFT+vdW scheme predicts not only a binding energy predominantly determined by vdW interactions between the base molecules and their substrates decreasing in the order of G>A>T>C, but also a very weak hybridization between the molecular levels of the nucleobases and the π-states of the BN sheet or graphene. This physisorption of G, A, T, and C on the BN sheet (graphene) induces a small interfacial dipole, giving rise to an energy shift in the work function by 0.11 (0.22), 0.09 (0.15), −0.05 (0.01), and 0.06 (0.13) eV, respectively.

Layered transition metal dichalcogenides MoTe2 and WTe2 share almost similar lattice constants as well as topological electronic properties except their structural phase transitions. While the former shows a first-order phase transition between monoclinic and orthorhombic structures, the latter does not. Using a recently proposed van der Waals density functional method, we investigate structural stability of the two materials and uncover that the disparate phase transitions originate from delicate differences between their interlayer bonding states near the Fermi energy. By exploiting the relation between the structural phase transitions and the low energy electronic properties, we show that a charge doping can control the transition substantially, thereby suggesting a way to stabilize or to eliminate their topological electronic energy bands. Because of the layered structures of TMDs, several polymorphs can exist and show characteristic physical properties depending on their structures 8 . A typical TMD shows the trigonal prismatic (2H) or the octahedral (1T ) structures 9-12 . For MoTe 2 and WTe 2 , the 2H structure (α-phase, P 6 3 /mmc) is a stable semiconductor while the 1T form is unstable 7,13 . The unstable 1T structure turns into the distorted octahedral one (1T ′ ) 7,14 . The stacked 1T ′ single layer forms a threedimensional bulk with the monoclinic structure (β-phase, P 2 1 /m) or the orthorhombic one (γ-phase, P mn2 1 ) (see Fig. 1) [15][16][17] . Interestingly, the β phase with a few layers is a potential candidate of QSH insulator 7 and the bulk γ phase shows type-II Weyl semimetalic energy bands 4,18,19 , respectively. Since the structural differences between β and γ phases are minute (∼4• tilting of axis along out-of-plane direction in β phase with respect to one in γ phase), the sensitive change in their topological low energy electronic properties is remarkable and the transition between different structures can lead to alternation of topological properties of the system.A phase transition between the β-and γ-phase in the layered TMDs has been known for a long time 16,20 . MoTe 2 shows a first-order transition from the β-to γ-structure at around 250 K 20 when temperature decreases. WTe 2 , however, does not show any transition and stays at the γ-phase 21,22 . Since the structural parameters of a single layer of 1T ′ -MoTe 2 and 1T ′ -WTe 2 are almost the same 15,17,23 and Mo and W belong to the same group in the periodic table, the different phase transition behaviors are intriguing and origins of the contrasting features are yet to be clarified. To understand the phase transition, the proper treatment of long and short range interlayer interaction in TMDs is essential. Most of the theoretical studies, however, fail to reproduce the experimental crystal structures of the two phases of MoTe 2 and WTe 2 so do their topological electronic structures using crystal structures obtained from ab initio calculations [24][25][26][27][28][29][30] . Instead, the atomic structures from experiment data are r...

Using first-principles density-functional theory (DFT) calculations, we investigate the 4/3-monolayer structure of Pb on the Si(111) or Ge(111) surface within the two competing structural models termed the H 3 and T 4 structures. We find that the spin-orbit coupling (SOC) influences the relative stability of the two structures in both the Pb/Si(111) and Pb/Ge(111) systems: i.e., our DFT calculation without including the SOC predicts that the T 4 structure is energetically favored over the H 3 structure by ∆E = 25 meV for Pb/Si(111) and 22 meV for Pb/Ge (111), but the inclusion of SOC reverses their relative stability as ∆E = −12 and −7 meV, respectively. Our analysis shows that the SOC-induced switching of the ground state is attributed to a more asymmetric surface charge distribution in the H 3 structure, which gives rise to a relatively larger Rashba spin splitting of surface states as well as a relatively larger pseudo-gap opening compared to the T 4 structure. By the nudged elastic band calculation, we obtain a sizable energy barrier from the H 3 to the T 4 structure as ∼0.59 and ∼0.27 eV for Pb/Si(111) and Pb/Ge(111), respectively. It is thus likely that the two energetically competing structures can coexist at low temperatures.

van der Waals energy-corrected density functional theory (DFT + vdW) as well as the exact exchange with electron correlation in the random-phase approximation are used to study the adsorption of benzene on the Si(001) surface with respect to two controversial adsorption structures (termed "butterfly" and "tight bridge"). Our finding that the tight-bridge structure is energetically favored over the butterfly structure agrees with standard DFT but conflicts with previous vdW-inclusive calculations. However, the inclusion of zero-point energy and thermal vibrations reverses the stability of the two structures with increasing temperature. Our results provide an explanation for the recent experimental observation that both structures coexist at room temperature. The interaction of aromatic hydrocarbon molecules with the Si(001) surface is of considerable interest because of the functionalization of the silicon surface toward molecular control of electronic devices. [1][2][3][4][5] Especially, the adsorption of benzene on the Si(001) surface has become a prototype system to study and understand the interaction of π -conjugated aromatic rings with the dangling bonds of the Si surface dimers. [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] Despite its apparent simplicity, the structure and dynamics of benzene on Si(001) have been controversial between experiment 6-14 and theory. [15][16][17][18][19] Many experimental studies on the adsorption of benzene on Si(001) have been performed at room temperature, using high-resolution electron-energy-loss spectroscopy, 6 thermaldesorption spectroscopy, 6,7 Auger electron spectroscopy, 6 near-edge x-ray-absorption fine structure, 8,9 and optical spectroscopy.10 All of them supported the so-called butterfly (BF) structure, where benzene adsorbs on top of a single Si dimer [see Fig. 1(a)]. However, scanning tunneling microscopy (STM) experiments at room temperature observed both the BF structure and the tight-bridge (TB) structure, where benzene adsorbs across two adjacent Si dimers [see Fig. 1(b)].11-13 It was also observed by STM that the BF structure was initially formed but was gradually converted to the TB structure, 12,13 implying that the BF structure is an intermediate state.Recently, a room-temperature photoelectron diffraction study concluded that the saturated monolayer (ML) of benzene on Si(001) contains both structures with 58 ± 29% of molecules having the BF structure. 14 To date, theoretical works remain divided on the issue of the adsorption structure of benzene on Si(001). [15][16][17][18][19] Standard density functional theory (DFT) calculations using the slab geometry predicted that the TB structure is more stable than the BF structure by 0.07-0.27 eV depending on the coverage, [15][16][17][18][19] and that the initially formed BF structure undergoes a conversion into the more stable TB structure with an activation barrier of ∼1 eV, 17 which seemed to be consistent with STM experiments.12,13 On the other hand, a MP2 cluster calculation (Møller-Plesset ...

Exploration and manipulation of electronic states in low-dimensional systems are of great importance in the fundamental and practical aspects of nanomaterial and nanotechnology. Here, we demonstrate that the incorporation of vacancy defects into monatomic indium wires on n-type Si(111) can stabilize electronically phase-separated ground states where the insulating 8×2 and metallic 4×1 phases coexist. Furthermore, the areal ratio of the two phases in the phase-separated states can be tuned reversibly by electric field or charge doping, and such tunabilities can be quantitatively captured by first principles-based modeling and simulations. The present results extend the realm of electronic phase separation from strongly correlated d-electron materials typically in bulk form to weakly interacting sp-electron systems in reduced dimensionality.