-
[show abstract]
[hide abstract]
ABSTRACT: We demonstrate the continuous tuning of the electronic structure of
atomically thin MoS2 on flexible substrates by applying a uniaxial tensile
strain. A redshift at a rate of ~70 meV per percent applied strain for direct
gap transitions, and at a rate 1.6 times larger for indirect gap transitions,
have been determined by absorption and photoluminescence spectroscopy. Our
result, in excellent agreement with first principles calculations, demonstrates
the potential of twodimensional crystals for applications in flexible
electronics and optoelectronics.
05/2013;
-
[show abstract]
[hide abstract]
ABSTRACT: Two-dimensional (2D) atomic crystals, such as graphene and transition-metal dichalcogenides, have emerged as a new class of materials with remarkable physical properties. In contrast to graphene, monolayer MoS(2) is a non-centrosymmetric material with a direct energy gap. Strong photoluminescence, a current on/off ratio exceeding 10(8) in field-effect transistors, and efficient valley and spin control by optical helicity have recently been demonstrated in this material. Here we report the spectroscopic identification in a monolayer MoS(2) field-effect transistor of tightly bound negative trions, a quasiparticle composed of two electrons and a hole. These quasiparticles, which can be optically created with valley and spin polarized holes, have no analogue in conventional semiconductors. They also possess a large binding energy (~ 20 meV), rendering them significant even at room temperature. Our results open up possibilities both for fundamental studies of many-body interactions and for optoelectronic and valleytronic applications in 2D atomic crystals.
Nature Material 12/2012; · 32.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Two-dimensional (2D) atomic crystals, such as graphene and transition-metal
dichalcogenides, have emerged as a new class of materials with remarkable
physical properties. In contrast to graphene, monolayer MoS2 is a
non-centrosymmetric material with a direct energy gap. Strong
photoluminescence, a current on-off ratio exceeding 10^8 in field-effect
transistors, and efficient valley and spin control by optical helicity have
recently been demonstrated in this material. Here we report the spectroscopic
identification in doped monolayer MoS2 of tightly bound negative trions, a
quasi-particle composed of two electrons and a hole. These quasi-particles,
which can be created with valley and spin polarized holes, have no analogue in
other semiconducting materials. They also possess a large binding energy (~ 20
meV), rendering them significant even at room temperature. Our results open up
new avenues both for fundamental studies of many-body interactions and for
opto-electronic and valleytronic applications in 2D atomic crystals.
10/2012;
-
Chris Ryan,
Cory W Christenson,
Brent Valle,
Anuj Saini,
Joseph Lott,
Jack Johnson,
David Schiraldi,
Christoph Weder,
Eric Baer,
Kenneth D Singer, Jie Shan
[show abstract]
[hide abstract]
ABSTRACT: 3D Optical data storage is demonstrated in co-extruded multilayer films using organic materials. Co-extrusion is able to produce films on a much larger scale at a much lower cost than current methods. The material compatibility and mechanical flexibility allow for new data formats with higher capacities to be realized.
Advanced Materials 07/2012; 24(38):5222-6. · 13.88 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Electronic and spintronic devices rely on the fact that free charge carriers in solids carry electric charge and spin. There are, however, other properties of charge carriers that might be exploited in new families of devices. In particular, if there are two or more minima in the conduction band (or maxima in the valence band) in momentum space, and if it is possible to confine charge carriers in one of these valleys, then it should be possible to make a valleytronic device. Valley polarization, as the selective population of one valley is designated, has been demonstrated using strain and magnetic fields, but neither of these approaches allows dynamic control. Here, we demonstrate that optical pumping with circularly polarized light can achieve complete dynamic valley polarization in monolayer MoS(2) (refs 11, 12), a two-dimensional non-centrosymmetric crystal with direct energy gaps at two valleys. Moreover, this polarization is retained for longer than 1 ns. Our results, and similar results by Zeng et al., demonstrate the viability of optical valley control and suggest the possibility of valley-based electronic and optoelectronic applications in MoS(2) monolayers.
Nature Nanotechnology 06/2012; 7(8):494-8. · 27.27 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The in-plane optical phonons around 200 meV in few-layer graphene are investigated utilizing infrared absorption spectroscopy. The phonon spectra exhibit unusual asymmetric features characteristic of Fano resonances, which depend critically on the layer thickness and stacking order of the sample. The phonon intensities in samples with rhombohedral (ABC) stacking are significantly higher than those with Bernal (AB) stacking. These observations reflect the strong coupling between phonons and interband electronic transitions in these systems and the distinctive variation in the joint density of electronic states in samples of differing thickness and stacking order.
Physical Review Letters 04/2012; 108(15):156801. · 7.37 Impact Factor
-
Advanced Materials 06/2011; 23(21):2425-9. · 13.88 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Significant excitonic effects were observed in graphene by measuring its optical conductivity in a broad spectral range including the two-dimensional π-band saddle-point singularities in the electronic structure. The strong electron-hole interactions manifest themselves in an asymmetric resonance peaked at 4.62 eV, which is redshifted by nearly 600 meV from the value predicted by ab initio GW calculations for the band-to-band transitions. The observed excitonic resonance is explained within a phenomenological model as a Fano interference of a strongly coupled excitonic state and a band continuum. Our experiment also showed a weak dependence of the excitonic resonance in few-layer graphene on layer thickness. This result reflects the effective cancellation of the increasingly screened repulsive electron-electron (e-e) and attractive electron-hole (e-h) interactions.
Physical Review Letters 01/2011; 106(4):046401. · 7.37 Impact Factor
-
Review of Modern Physics 01/2011; 83:543. · 43.93 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Since graphene has no band gap, photoluminescence is not expected from relaxed charge carriers. We have, however, observed significant light emission from graphene under excitation by ultrashort (30-fs) laser pulses. Light emission was found to occur across the visible spectral range (1.7-3.5 eV), with emitted photon energies exceeding that of the excitation laser (1.5 eV). The emission exhibits a nonlinear dependence on the laser fluence. In two-pulse correlation measurements, a dominant relaxation time of tens of femtoseconds is observed. A two-temperature model describing the electrons and their interaction with strongly coupled optical phonons can account for the experimental observations.
Physical Review Letters 09/2010; 105(12):127404. · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The electronic properties of ultrathin crystals of molybdenum disulfide consisting of N=1,2,…,6 S-Mo-S monolayers have been investigated by optical spectroscopy. Through characterization by absorption, photoluminescence, and photoconductivity spectroscopy, we trace the effect of quantum confinement on the material's electronic structure. With decreasing thickness, the indirect band gap, which lies below the direct gap in the bulk material, shifts upwards in energy by more than 0.6 eV. This leads to a crossover to a direct-gap material in the limit of the single monolayer. Unlike the bulk material, the MoS₂ monolayer emits light strongly. The freestanding monolayer exhibits an increase in luminescence quantum efficiency by more than a factor of 10⁴ compared with the bulk material.
Physical Review Letters 09/2010; 105(13):136805. · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The electronic structure of few-layer graphene (FLG) samples with crystalline order was investigated experimentally by infrared absorption spectroscopy for photon energies ranging from 0.2-1 eV. Distinct optical conductivity spectra were observed for different samples having precisely the same number of layers. The different spectra arise from the existence of two stable polytypes of FLG, namely, Bernal (AB) stacking and rhombohedral (ABC) stacking. The observed absorption features, reflecting the underlying symmetry of the two polytypes and the nature of the associated van Hone singularities, were reproduced by explicit calculations within a tight-binding model. The findings demonstrate the pronounced effect of stacking order on the electronic structure of FLG.
Physical Review Letters 04/2010; 104(17):176404. · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: It has been predicted that application of a strong electric field perpendicular to the plane of bilayer graphene can induce a significant band gap. We have measured the optical conductivity of bilayer graphene with an efficient electrolyte top gate for a photon energy range of 0.2-0.7 eV. We see the emergence of new transitions as a band gap opens. A band gap approaching 200 meV is observed when an electric field approximately 1 V/nm is applied, inducing a carrier density of about 10(13) cm(-2)}. The magnitude of the band gap and the features observed in the infrared conductivity spectra are broadly compatible with calculations within a tight-binding model.
Physical Review Letters 07/2009; 102(25):256405. · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We have developed a general analytic description of polarized light pulses and explored the properties of circularly polarized single-cycle pulses. The temporal evolution of the electric-field vector of such spectrally broad pulses, which may be described in terms of a Hilbert transform relationship, differs significantly from the well-known behavior of quasi-monochromatic radiation. Single-cycle circularly polarized pulses are produced and characterized experimentally in the terahertz spectral region.
Optics Express 05/2009; 17(9):7431-9. · 3.59 Impact Factor
-
Advanced Materials 08/2008; 20(19):3649 - 3653. · 13.88 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Dependence of the polarizability of a QD on the number of excitons in it was investigated. A simple model of weakly interacting charge carriers in an infinite spherical potential well agrees well with the experiment.
Quantum Electronics and Laser Science Conference, 2007. QELS '07; 06/2007
-
[show abstract]
[hide abstract]
ABSTRACT: Terahertz time-domain spectroscopy was employed to investigate photogenerated excitons in PbSe and CdSe quantum dots (QD) of radii less than their exciton Bohr radius. The exciton response was found to be “atom”-like and rise with increasing the carrier effective mass. Excitons in CdSe were found to be about two times more polarizable than those in PbSe QDs of equal size. These experimental findings including both the magnitude of the exciton polarizability and its variation with the QD radius were well described by an effective mass model with multiband structures of the QD materials.
03/2007;
-
[show abstract]
[hide abstract]
ABSTRACT: The response of charge to externally applied electric fields is an important basic property of any material system, as well as one critical for many applications. Here, we examine the behaviour and dynamics of charges fully confined on the nanometre length scale. This is accomplished using CdSe nanocrystals of controlled radius (1-2.5 nm) as prototype quantum systems. Individual electron-hole pairs are created at room temperature within these structures by photoexcitation and are probed by terahertz (THz) electromagnetic pulses. The electronic response is found to be instantaneous even for THz frequencies, in contrast to the behaviour reported in related measurements for larger nanocrystals and nanocrystal assemblies. The measured polarizability of an electron-hole pair (exciton) amounts to approximately 10(4) A(3) and scales approximately as the fourth power of the nanocrystal radius. This size dependence and the instantaneous response reflect the presence of well-separated electronic energy levels induced in the system by strong quantum-confinement effects.
Nature Material 12/2006; 5(11):861-4. · 32.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We present investigations of the transient photoconductivity and recombination dynamics of quasifree electrons in liquid n-hexane and cyclohexane performed using terahertz time-domain spectroscopy (THz-TDS). Quasifree electrons are generated by two-photon photoionization of the liquid using a femtosecond ultraviolet pulse, and the resulting changes in the complex conductivity are probed by a THz electromagnetic pulse at a variable delay. The detection of time-domain wave forms of the THz electric field permits the direct determination of both the real and the imaginary part of the conductivity of the electrons over a wide frequency range. The change in conductivity can be described by the Drude model, thus yielding the quasifree electron density and scattering time. The electron density is found to decay on a time scale of a few hundred picoseconds, which becomes shorter with increasing excitation density. The dynamics can be described by a model that assumes nongeminate recombination between electrons and positive ions. In addition, a strong dependence of the quasifree electron density on temperature is observed, in agreement with a two-state model in which the electron may exist in either a quasifree or a bound state.
The Journal of Chemical Physics 08/2004; 121(1):394-404. · 3.33 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: THz time-domain spectroscopy (THz TDS) with ultrafast photo-excitation is applied to probe the complex cond. of the charge carriers in sapphire over the temp. range of 40-350 K. A comparison of the measured complex cond. to the Drude model yields the carrier scattering rate and d. The dependence of the carrier scattering rate on temp. and sample purity is used to identify the scattering mechanisms in sapphire. In the higher temp. range, scattering is detd. by intrinsic phonon processes, but impurity scattering becomes dominant at low temps. in typical optical-grade samples. In high-purity samples, however, impurity scattering remains negligible down to 40 K, and carrier mobilities exceeding 10,000 cm2/Vscan be achieved. [on SciFinder (R)]
Proceedings of SPIE-The International Society for Optical Engineering. 01/2004; 5352:216-221.
