201 research outputs found
Optical signature of symmetry variations and spin-valley coupling in atomically thin tungsten dichalcogenides
Motivated by the triumph and limitation of graphene for electronic
applications, atomically thin layers of group VI transition metal
dichalcogenides are attracting extensive interest as a class of graphene-like
semiconductors with a desired band-gap in the visible frequency range. The
monolayers feature a valence band spin splitting with opposite sign in the two
valleys located at corners of 1st Brillouin zone. This spin-valley coupling,
particularly pronounced in tungsten dichalcogenides, can benefit potential
spintronics and valleytronics with the important consequences of spin-valley
interplay and the suppression of spin and valley relaxations. Here we report
the first optical studies of WS2 and WSe2 monolayers and multilayers. The
efficiency of second harmonic generation shows a dramatic even-odd oscillation
with the number of layers, consistent with the presence (absence) of inversion
symmetry in even-layer (odd-layer). Photoluminescence (PL) measurements show
the crossover from an indirect band gap semiconductor at mutilayers to a
direct-gap one at monolayers. The PL spectra and first-principle calculations
consistently reveal a spin-valley coupling of 0.4 eV which suppresses
interlayer hopping and manifests as a thickness independent splitting pattern
at valence band edge near K points. This giant spin-valley coupling, together
with the valley dependent physical properties, may lead to rich possibilities
for manipulating spin and valley degrees of freedom in these atomically thin 2D
materials
Charge photogeneration in few-layer MoS2
The two-dimensional semiconductor MoS2 in its mono- and few-layer form is
expected to have a significant exciton binding energy of several 100 meV,
leading to the consensus that excitons are the primary photoexcited species.
Nevertheless, even single layers show a strong photovoltaic effect and work as
the active material in high sensitivity photodetectors, thus indicating
efficient charge carrier photogeneration (CPG). Here we use continuous wave
photomodulation spectroscopy to identify the optical signature of long-lived
charge carriers and femtosecond pump-probe spectroscopy to follow the CPG
dynamics. We find that intitial photoexcitation yields a branching between
excitons and charge carriers, followed by excitation energy dependent hot
exciton dissociation as an additional CPG mechanism. Based on these findings,
we make simple suggestions for the design of more efficient MoS2 photovoltaic
and photodetector devices
Valence band structure of polytypic zinc-blende/wurtzite GaAs nanowires probed by polarization-dependent photoluminescence
We conducted temperature-dependent measurements of the photoluminescence (PL) polarization on GaAs nanowires (NWs) with polytypic zinc-blende/wurtzite structure in order to probe the symmetry and energy structure of the valence band in the wurtzite segments of the NWs. The low-temperature measurements revealed that in most of the investigated cases, the ground level of the interface excitons responsible for the PL is formed by the heavy hole. To describe the observed temperature dependence of the degree of PL polarization, we developed a theoretical model that allows an estimation of the splitting between the heavy hole and light hole exciton subbands in these NWs. This splitting is smaller than expected in pure wurtzite on the basis of recent first-principles calculations, which may be attributed to the multiple twinned nature of the wurtzite sections, which effectively behave as a polytype of lower hexagonality
Computing Optical Properties of Ultra-thin Crystals
An overview is given of recent advances in experimental and theoretical
understanding of optical properties of ultra-thin crystal structures (graphene,
phosphorene, silicene, MoS2, MoSe2 , WS2 , WSe2 , h-AlN, h-BN, fluorographene,
graphane). Ultra-thin crystals are atomically-thick layered crystals that have
unique properties which differ from their 3D counterpart. Because of the
difficulties in the synthesis of few-atom-thick crystal structures, which are
thought to be the main building blocks of future nanotechnology, reliable
theoretical predictions of their electronic, vibrational and optical properties
are of great importance. Recent studies revealed the reliable predictive power
of existing theoretical approaches based on density functional theory (DFT)
Temperature induced crossing in the optical bandgap of mono and bilayer MoS2 on SiO2
Photoluminescence measurements in mono- and bilayer-MoS2 on SiO2 were undertaken to determine the thermal effect of the MoS2/SiO2 interface on the optical bandgap. The energy and intensity of the photoluminescence from monolayer MoS2 were lower and weaker than those from bilayer MoS2 at low temperatures, whilst the opposite was true at high temperatures above 200 K. Density functional theory calculations suggest that the observed optical bandgap crossover is caused by a weaker substrate coupling to the bilayer than to the monolayer
Phonon-driven spin-Floquet magneto-valleytronics in MoS2
Two-dimensional materials equipped with strong spin-orbit coupling can display novel electronic, spintronic, and topological properties originating from the breaking of time or inversion symmetry. A lot of interest has focused on the valley degrees of freedom that can be used to encode binary information. By performing ab initio time-dependent density functional simulation on MoS2, here we show that the spin is not only locked to the valley momenta but strongly coupled to the optical E '' phonon that lifts the lattice mirror symmetry. Once the phonon is pumped so as to break time-reversal symmetry, the resulting Floquet spectra of the phonon-dressed spins carry a net out-of-plane magnetization (approximate to 0.024 mu(B) for single-phonon quantum) even though the original system is non-magnetic. This dichroic magnetic response of the valley states is general for all 2H semiconducting transition-metal dichalcogenides and can be probed and controlled by infrared coherent laser excitation
Bright excitons in monolayer transition metal dichalcogenides: from Dirac cones to Dirac saddle points
In monolayer transition metal dichalcogenides, tightly bound excitons have
been discovered with a valley pseudospin that can be optically addressed
through polarization selection rules. Here, we show that this valley pseudospin
is strongly coupled to the exciton center-of-mass motion through electron-hole
exchange. This coupling realizes a massless Dirac cone with chirality index I=2
for excitons inside the light cone, i.e. bright excitons. Under moderate
strain, the I=2 Dirac cone splits into two degenerate I=1 Dirac cones, and
saddle points with a linear Dirac spectrum emerge in the bright exciton
dispersion. Interestingly, after binding an extra electron, the charged exciton
becomes a massive Dirac particle associated with a large valley Hall effect
protected from intervalley scattering. Our results point to unique
opportunities to study Dirac physics, with exciton's optical addressability at
specifiable momentum, energy and pseudospin. The strain-tunable valley-orbit
coupling also implies new structures of exciton condensates, new
functionalities of excitonic circuits, and possibilities for mechanical control
of valley pseudospin
Vacancy and Doping States in Monolayer and bulk Black Phosphorus.
The atomic geometries and transition levels of point defects and substitutional dopants in few-layer and bulk black phosphorus are calculated. The vacancy is found to reconstruct in monolayer P to leave a single dangling bond, giving a negative U defect with a +/- transition level at 0.24 eV above the valence band edge. The V(-) state forms an unusual 4-fold coordinated site. In few-layer and bulk black P, the defect becomes a positive U site. The divacancy is much more stable than the monovacancy, and it reconstructs to give no deep gap states. Substitutional dopants such as C, Si, O or S do not give rise to shallow donor or acceptor states but instead reconstruct to form non-doping sites analogous to DX or AX centers in GaAs. Impurities on black P adopt the 8-N rule of bonding, as in amorphous semiconductors, rather than simple substitutional geometries seen in tetrahedral semiconductors
Electrical Tuning of Valley Magnetic Moment via Symmetry Control
Crystal symmetry governs the nature of electronic Bloch states. For example,
in the presence of time reversal symmetry, the orbital magnetic moment and
Berry curvature of the Bloch states must vanish unless inversion symmetry is
broken. In certain 2D electron systems such as bilayer graphene, the intrinsic
inversion symmetry can be broken simply by applying a perpendicular electric
field. In principle, this offers the remarkable possibility of switching on/off
and continuously tuning the magnetic moment and Berry curvature near the Dirac
valleys by reversible electrical control. Here we demonstrate this principle
for the first time using bilayer MoS2, which has the same symmetry as bilayer
graphene but has a bandgap in the visible that allows direct optical probing of
these Berry-phase related properties. We show that the optical circular
dichroism, which reflects the orbital magnetic moment in the valleys, can be
continuously tuned from -15% to 15% as a function of gate voltage in bilayer
MoS2 field-effect transistors. In contrast, the dichroism is gate-independent
in monolayer MoS2, which is structurally non-centrosymmetric. Our work
demonstrates the ability to continuously vary orbital magnetic moments between
positive and negative values via symmetry control. This represents a new
approach to manipulating Berry-phase effects for applications in quantum
electronics associated with 2D electronic materials.Comment: 13 pages main text + 4 pages supplementary material
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