22 research outputs found
Does the Dirac Cone Exist in Silicene on Metal Substrates?
Absence of the Dirac cone due to a strong band hybridization is revealed to
be a common feature for epitaxial silicene on metal substrates according to our
first-principles calculations for silicene on Ir, Cu, Mg, Au, Pt, Al, and Ag
substrates. The destroyed Dirac cone of silicene, however, can be effectively
restored with linear or parabolic dispersion by intercalating alkali metal
atoms between silicene and the metal substrates, offering an opportunity to
study the intriguing properties of silicene without further transfer of
silicene from the metal substrates
新奇2次元材料の表面/界面最適化と物性解析に関する理論研究
学位の種別: 課程博士審査委員会委員 : (主査)東京大学教授 渡邉 聡, 東京大学教授 町田 友樹, 東京大学教授 押山 淳, 東京大学准教授 長汐 晃輔, 東京大学准教授 高木 紀明, 東京大学講師 南谷 英美University of Tokyo(東京大学
Silicene Nanomesh
Similar to graphene, zero band gap limits the application of silicene in
nanoelectronics despite of its high carrier mobility. By using first-principles
calculations, we reveal that a band gap is opened in silicene nanomesh (SNM)
when the width W of the wall between the neighboring holes is even. The size of
the band gap increases with the reduced W and has a simple relation with the
ratio of the removed Si atom and the total Si atom numbers of silicene. Quantum
transport simulation reveals that the sub-10 nm single-gated SNM field effect
transistors show excellent performance at zero temperature but such a
performance is greatly degraded at room temperature
Interfacial Properties of Bilayer and Trilayer Graphene on Metal Substrates
One popular approach to prepare graphene is to grow them on transition metal
substrates via chemical vapor deposition. By using the density functional
theory with dispersion correction, we systematically investigate for the first
time the interfacial properties of bilayer (BLG) and trilayer graphene (TLG) on
metal substrates. Three categories of interfacial structures are revealed. The
adsorption of B(T)LG on Al, Ag, Cu, Au, and Pt substrates is a weak
physisorption, but a band gap can be opened. The adsorption of B(T)LG on Ti,
Ni, and Co substrates is a strong chemisorption, and a stacking-insensitive
band gap is opened for the two uncontacted layers of TLG. The adsorption of
B(T)LG on Pd substrate is a weaker chemisorption, with a band gap opened for
the uncontacted layers. This fundamental study also helps for B(T)LG device
study due to inevitable graphene/metal contact.Comment: 1 table, 8 figure
Interfacial Properties of Monolayer and Bilayer MoS2 Contacts with Metals: Beyond the Energy Band Calculations
Although many prototype devices based on two-dimensional (2D) MoS2 have been
fabricated and wafer scale growth of 2D MoS2 has been realized, the fundamental
nature of 2D MoS2-metal contacts has not been well understood yet. We provide a
comprehensive ab initio study of the interfacial properties of a series of
monolayer (ML) and bilayer (BL) MoS2-metal contacts (metal = Sc, Ti, Ag, Pt,
Ni, and Au). A comparison between the calculated and observed Schottky barrier
heights (SBHs) suggests that many-electron effects are strongly suppressed in
channel 2D MoS2 due to a charge transfer. The extensively adopted energy band
calculation scheme fails to reproduce the observed SBHs in 2D MoS2-Sc
interface. By contrast, an ab initio quantum transport device simulation better
reproduces the observed SBH in the two types of contacts and highlights the
importance of a higher level theoretical approach beyond the energy band
calculation in the interface study. BL MoS2-metal contacts have a reduced SBH
than ML MoS2-metal contacts due to the interlayer coupling and thus have a
higher electron injection efficiency.Comment: 36 pages, 13 figures, 3 table
Does P-type Ohmic Contact Exist in WSe2-metal Interfaces?
Formation of low-resistance metal contacts is the biggest challenge that
masks the intrinsic exceptional electronic properties of 2D WSe2 devices. We
present the first comparative study of the interfacial properties between ML/BL
WSe2 and Sc, Al, Ag, Au, Pd, and Pt contacts by using ab initio energy band
calculations with inclusion of the spin-orbital coupling (SOC) effects and
quantum transport simulations. The interlayer coupling tends to reduce both the
electron and hole Schottky barrier heights (SBHs) and alters the polarity for
WSe2-Au contact, while the SOC chiefly reduces the hole SBH. In the absence of
the SOC, Pd contact has the smallest hole SBH with a value no less than 0.22
eV. Dramatically, Pt contact surpasses Pd contact and becomes p-type Ohmic or
quasi-Ohmic contact with inclusion of the SOC. Our study provides a theoretical
foundation for the selection of favorable metal electrodes in ML/BL WSe2
devices
Half-Metallic Silicene and Germanene Nanoribbons: towards High-Performance Spintronics Device
By using first-principles calculations, we predict that an in-plane
homogenous electrical field can induce half-metallicity in hydrogen-terminated
zigzag silicene and germanene nanoribbons (ZSiNRs and ZGeNRs). A dual-gated
finite ZSiNR device reveals a nearly perfect spin-filter efficiency of up to
99% while a quadruple-gated finite ZSiNR device serves as an effective spin
field effect transistor (FET) with an on/off current ratio of over 100 from ab
initio quantum transport simulation. This discovery opens up novel prospect of
silicene and germanene in spintronics
Tunable band gap in few-layer graphene by surface adsorption
There is a tunable band gap in ABC-stacked few-layer graphene (FLG) via
applying a vertical electric field, but the operation of FLG-based field effect
transistor (FET) requires two gates to create a band gap and tune channel's
conductance individually. Using first principle calculations, we propose an
alternative scheme to open a band gap in ABC-stacked FLG namely via single-side
adsorption. The band gap is generally proportional to the charge transfer
density. The capability to open a band gap of metal adsorption decreases in
this order: K/Al > Cu/Ag/Au > Pt. Moreover, we find that even the band gap of
ABA-stacked FLG can be opened if the bond symmetry is broken. Finally, a
single-gated FET based on Cu-adsorbed ABC-stacked trilayer graphene is
simulated. A clear transmission gap is observed, which is comparable with the
band gap. This renders metal-adsorbed FLG a promising channel in a single-gated
FET device