The electronic and optical properties and the recent progress in applications of 2D semiconductor transition metal dichalcogenides with emphasis on strong excitonic effects, and spin- and valley-dependent properties are reviewed. Recent advances in the development of atomically thin layers of van der Waals bonded solids have opened up new possibilities for the exploration of 2D physics as well as for materials for applications. Among them, semiconductor transition metal dichalcogenides, MX2 (M = Mo, W; X = S, Se), have bandgaps in the near-infrared to the visible region, in contrast to the zero bandgap of graphene. In the monolayer limit, these materials have been shown to possess direct bandgaps, a property well suited for photonics and optoelectronics applications. Here, we review the electronic and optical properties and the recent progress in applications of 2D semiconductor transition metal dichalcogenides with emphasis on strong excitonic effects, and spin- and valley-dependent properties.

Photonics and optoelectronics of 2D semiconductor transition metal dichalcogenides

During the last few years the developments in the field of III–nitrides have been spectacular. High quality epitaxial layers can now be grown by MOVPE. Recently good quality epilayers have also been grown by MBE. Considerable work has been done on dislocations, strain, and critical thickness of GaN grown on different substrates. Splitting of valence band by crystal field and by spin-orbit interaction has been calculated and measured. The measured values agree with the calculated values. Effects of strain on the splitting of the valence band and on the optical properties have been studied in detail. Values of band offsets at the heterointerface between several pairs of different nitrides have been determined. Extensive work has been done on the optical and electrical properties. Near band-edge spectra have been measured over a wide range of temperatures. Free and bound exciton peaks have been resolved. Valence band structure has been determined using the PL spectra and compared with the theoretically calcu...

III–nitrides: Growth, characterization, and properties

The syntheses of strongly anisotropic nanocrystals with one dimension much smaller than the two others, such as nanoplatelets, are still greatly underdeveloped. Here, we demonstrate the formation of atomically flat quasi-two-dimensional colloidal CdSe, CdS and CdTe nanoplatelets with well-defined thicknesses ranging from 4 to 11 monolayers. These nanoplatelets have the electronic properties of two-dimensional quantum wells formed by molecular beam epitaxy, and their thickness-dependent absorption and emission spectra are described very well within an eight-band Pidgeon-Brown model. They present an extremely narrow emission spectrum with full-width at half-maximum less than 40 meV at room temperature. The radiative fluorescent lifetime measured in CdSe nanoplatelets decreases with temperature, reaching 1 ns at 6 K, two orders of magnitude less than for spherical CdSe nanoparticles. This makes the nanoplatelets the fastest colloidal fluorescent emitters and strongly suggests that they show a giant oscillator strength transition.

Colloidal nanoplatelets with two-dimensional electronic structure

We report on a strong photoluminescence (PL) enhancement of monolayer MoS2 through defect engineering and oxygen bonding. Micro-PL and Raman images clearly reveal that the PL enhancement occurs at cracks/defects formed during high-temperature annealing. The PL enhancement at crack/defect sites could be as high as thousands of times after considering the laser spot size. The main reasons of such huge PL enhancement include the following: (1) the oxygen chemical adsorption induced heavy p doping and the conversion from trion to exciton; (2) the suppression of nonradiative recombination of excitons at defect sites, which was verified by low-temperature PL measurements. First-principle calculations reveal a strong binding energy of ∼2.395 eV for an oxygen molecule adsorbed on a S vacancy of MoS2. The chemically adsorbed oxygen also provides a much more effective charge transfer (0.997 electrons per O2) compared to physically adsorbed oxygen on an ideal MoS2 surface. We also demonstrate that the defect enginee...

Strong Photoluminescence Enhancement of MoS2 through Defect Engineering and Oxygen Bonding

The electronic excited states of molecular aggregates and their photophysical signatures have long fascinated spectroscopists and theoreticians alike since the advent of Frenkel exciton theory almost 90 years ago. The influence of molecular packing on basic optical probes like absorption and photoluminescence was originally worked out by Kasha for aggregates dominated by Coulombic intermolecular interactions, eventually leading to the classification of J- and H-aggregates. This review outlines advances made in understanding the relationship between aggregate structure and photophysics when vibronic coupling and intermolecular charge transfer are incorporated. An assortment of packing geometries is considered from the humble molecular dimer to more exotic structures including linear and bent aggregates, two-dimensional herringbone and “HJ” aggregates, and chiral aggregates. The interplay between long-range Coulomb coupling and short-range charge-transfer-mediated coupling strongly depends on the aggregate ...

Expanded Theory of H- and J-Molecular Aggregates: The Effects of Vibronic Coupling and Intermolecular Charge Transfer.

The fundamental relationship between radiative lifetime and spectral linewidth of free excitons is demonstrated theoretically and experimentally for quasi 2D excitons in GaAs/AlGaAs quantum wells.

Linewidth dependence of radiative exciton lifetimes in quantum wells

Charge-transfer transitions in crystalline anthracene and their role in photoconductivity

A detailed experimental study of the real-space \ensuremath{\Gamma}-X transfer in type-II GaAs/AlAs short-period superlattices and in type-II ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/AlAs multiple-quantum-well structures is presented. Transfer times on a subpicosecond and picosecond time scale are observed. The time constants critically depend on the thickness of the (Al,Ga)As layers, but not on the AlAs-layer thickness in the samples studied. The \ensuremath{\Gamma}-X transfer rate is determined by the spatial overlap of the \ensuremath{\Gamma} and X wave functions confined in the different layers. Intensity- and temperature-dependent measurements provide insight into the scattering mechanism. We conclude that electron--LO-phonon scattering is the dominant scattering process for samples with thick (Al,Ga)As layers (g100 \AA{}). In contrast, interface scattering due to the interface mixing potential (\ensuremath{\Gamma}-${\mathit{X}}_{\mathit{z}}$ mixing) and/or due to potential fluctuations caused by interface roughness (\ensuremath{\Gamma}-${\mathit{X}}_{\mathit{x},}$y mixing) probably dominates for samples with thin (Al,Ga)As layers (35 \AA{}).

Experimental study of the Γ-X electron transfer in type-II (Al,Ga)As/AlAs superlattices and multiple-quantum-well structures

The trapping efficiency and trapping dynamics of photoexcited carriers in GaAs/AlGaAs single quantum wells with different confinement structures are examined at low temperature by means of picosecond luminescence as well as photoluminescence excitation spectroscopy. The trapping efficiency is 100% only in graded‐index separate confinement heterostructures with a linear band‐gap profile. The lower trapping efficiency of other confinement structures is due to radiative and nonradiative recombination in the confinement layers.

Carrier trapping in single quantum wells with different confinement structures

This paper reports detailed experimental studies of low-temperature carrier trapping in GaAs/${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As single quantum wells with 5 nm and 1.2 nm thickness, respectively, with different confinement structures. Trapping efficiency and trapping dynamics are studied by means of photoluminescence, photoluminescence excitation spectroscopy, and picosecond luminescence spectroscopy. We obtain trapping efficiencies of about 40% for both the single quantum wells without additional confinement and separate-confinement-heterostructure quantum wells. The percentage of trapped carriers increases to about 60% to 80% for quantum wells cladded by graded-index separate-confinement heterostructures with parabolic band-gap profile. The maximum trapping efficiency of about 100% has been observed for a separate-confinement heterostructure with a linear band-gap profile. Interlayer well width fluctuations are found to be unimportant for the trapping behavior in our samples. Trapping times are appreciably shorter than the rise times of the quantum well photoluminescence, which are between 60 and 100 ps for the different structures. Surface recombination in the 0.2-\ensuremath{\mu}m-thick ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As cladding layer does not reduce the trapping efficiency of the single quantum well without additional confinement compared with the separate-confinement heterostructure. An effective trapping area with about 80 nm width can be deduced on the basis of these results for the quantum-well structures with ungraded cladding layers.

G. Peter

Papers

Linewidth dependence of radiative exciton lifetimes in quantum wells

Charge-transfer transitions in crystalline anthracene and their role in photoconductivity

Experimental study of the Γ-X electron transfer in type-II (Al,Ga)As/AlAs superlattices and multiple-quantum-well structures

Carrier trapping in single quantum wells with different confinement structures

Trapping of carriers in single quantum wells with different configurations of the confinement layers.