Showing papers on "Exciton published in 2015"
TL;DR: Efficient organic-inorganic perovskite light-emitting diodes were made with nanograin crystals that lack metallic lead, which helped to confine excitons and avoid their quenching.
Abstract: Organic-inorganic hybrid perovskites are emerging low-cost emitters with very high color purity, but their low luminescent efficiency is a critical drawback. We boosted the current efficiency (CE) of perovskite light-emitting diodes with a simple bilayer structure to 42.9 candela per ampere, similar to the CE of phosphorescent organic light-emitting diodes, with two modifications: We prevented the formation of metallic lead (Pb) atoms that cause strong exciton quenching through a small increase in methylammonium bromide (MABr) molar proportion, and we spatially confined the exciton in uniform MAPbBr3 nanograins (average diameter = 99.7 nanometers) formed by a nanocrystal pinning process and concomitant reduction of exciton diffusion length to 67 nanometers. These changes caused substantial increases in steady-state photoluminescence intensity and efficiency of MAPbBr3 nanograin layers.
2,295 citations
TL;DR: It is shown that a large delocalization of the highest occupied molecular orbital and lowest unoccupied molecular orbital in these charge-transfer compounds enhances the rate of radiative decay considerably by inducing a large oscillator strength even when there is a small overlap between the two wavefunctions.
Abstract: Organic compounds that exhibit highly efficient, stable blue emission are required to realize inexpensive organic light-emitting diodes for future displays and lighting applications. Here, we define the design rules for increasing the electroluminescence efficiency of blue-emitting organic molecules that exhibit thermally activated delayed fluorescence. We show that a large delocalization of the highest occupied molecular orbital and lowest unoccupied molecular orbital in these charge-transfer compounds enhances the rate of radiative decay considerably by inducing a large oscillator strength even when there is a small overlap between the two wavefunctions. A compound based on our design principles exhibited a high rate of fluorescence decay and efficient up-conversion of triplet excitons into singlet excited states, leading to both photoluminescence and internal electroluminescence quantum yields of nearly 100%.
1,007 citations
TL;DR: In this article, a comprehensive first-principles study of the electronic structure of 51 semiconducting monolayer transition-metal dichalcogenides and -oxides in the 2H and 1T hexagonal phases is presented.
Abstract: We present a comprehensive first-principles study of the electronic structure of 51 semiconducting monolayer transition-metal dichalcogenides and -oxides in the 2H and 1T hexagonal phases. The quasiparticle (QP) band structures with spin–orbit coupling are calculated in the G0W0 approximation, and comparison is made with different density functional theory descriptions. Pitfalls related to the convergence of GW calculations for two-dimensional (2D) materials are discussed together with possible solutions. The monolayer band edge positions relative to vacuum are used to estimate the band alignment at various heterostructure interfaces. The sensitivity of the band structures to the in-plane lattice constant is analyzed and rationalized in terms of the electronic structure. Finally, the q-dependent dielectric functions and effective electron and hole masses are obtained from the QP band structure and used as input to a 2D hydrogenic model to estimate exciton binding energies. Throughout the paper we focus on...
892 citations
TL;DR: A thermally activated delayed fluorescence material for organic light-emitting diodes is shown, which realizes both approximately 100% photoluminescence quantum yield and Approximately 100% up-conversion of the triplet to singlet excited state.
Abstract: Efficient organic light-emitting diodes have been developed using emitters containing rare metals, such as platinum and iridium complexes. However, there is an urgent need to develop emitters composed of more abundant materials. Here we show a thermally activated delayed fluorescence material for organic light-emitting diodes, which realizes both approximately 100% photoluminescence quantum yield and approximately 100% up-conversion of the triplet to singlet excited state. The material contains electron-donating diphenylaminocarbazole and electron-accepting triphenyltriazine moieties. The typical trade-off between effective emission and triplet-to-singlet up-conversion is overcome by fine-tuning the highest occupied molecular orbital and lowest unoccupied molecular orbital distributions. The nearly zero singlet–triplet energy gap, smaller than the thermal energy at room temperature, results in an organic light-emitting diode with external quantum efficiency of 29.6%. An external quantum efficiency of 41.5% is obtained when using an out-coupling sheet. The external quantum efficiency is 30.7% even at a high luminance of 3,000 cd m−2. Organic light-emitting diodes promise a more environment-friendly future for light sources, but many use rare metals. Here, the authors present an approach that achieves external quantum efficiency over 40% by realising 100% up-conversion from triplet to singlet excitons and thus 100% radiative emission.
718 citations
TL;DR: In this paper, the authors provide a basic physical description of the exciton diffusion in organic semiconductors and present experimental methods that are used to measure the key parameters of this process.
Abstract: The purpose of this review is to provide a basic physical description of the exciton diffusion in organic semiconductors. Furthermore, experimental methods that are used to measure the key parameters of this process as well as strategies to manipulate the exciton diffusion length are summarized. Special attention is devoted to the temperature dependence of exciton diffusion and its relationship to Forster energy transfer rates. An extensive table of more than a hundred measurements of the exciton diffusion length in various organic semiconductors is presented. Finally, an outlook of remaining challenges for future research is provided.
659 citations
TL;DR: In this paper, the authors studied how changes in the structural features of poly(3-hexylthiophene (P3HT) polymers affect exciton dissociation processes and concluded that excitons in disordered regions between crystalline and amorphous phases dissociate extrinsically with yield and spatial distribution.
Abstract: The optoelectronic properties of macromolecular semiconductors depend fundamentally on their solid-state microstructure and phase morphology. Hence, it is of central importance to manipulate—from the outset—the molecular arrangement and packing of this special class of polymers from the nano- to the micrometer scale when they are integrated in thin film devices such as photovoltaic cells, transistors or light-emitting diodes, for example. One effective strategy for this purpose is to vary their molecular weight. The reason for this is that materials of different weight-average molecular weight (Mw) lead to different microstructures. Polymers of low Mw form unconnected, extended-chain crystals because of their non-entangled nature. As a result, a polycrystalline, one-phase morphology is obtained. In contrast, high-Mw materials, in which average chain lengths are longer than the length between entanglements, form two-phase morphologies comprised of crystalline moieties embedded in largely un-ordered (amorphous) regions. Here, we discuss how changes in these structural features affect exciton dissociation processes. We utilise neat regioregular poly(3-hexylthiophene) (P3HT) of varying Mw as a model system and apply time-resolved photoluminescence (PL) spectroscopy to probe the electronic landscape in a range of P3HT thin-film architectures. We find that at 10 K, PL originating from recombination of long-lived charge pairs decays over microsecond timescales. Tellingly, both the amplitude and decay-rate distribution depend strongly on Mw. In films with dominant one-phase, chain-extended microstructures, the delayed PL is suppressed as a result of a diminished yield of photoinduced charges. Its decay is significantly slower than in two-phase microstructures. We therefore conclude that excitons in disordered regions between crystalline and amorphous phases dissociate extrinsically with yield and spatial distribution that depend intimately upon microstructure, in agreement with previous work [Paquin et al., Phys. Rev. Lett., 2011, 106, 197401]. We note, however, that independent of Mw, the delayed-PL lineshape due to charge recombination is representative of that in low-Mw microstructures. We thus hypothesize that charge recombination at these low temperatures—and likely also charge generation—occur in torsionally disordered chains forming more strongly coupled photophysical aggregates than those in the steady-state ensemble, producing a delayed PL lineshape reminiscent of that in paraffinic morphologies at steady state.
580 citations
TL;DR: The wide radiative lifetime tunability, together with the ability shown here to predict radiative lifetimes from computations, hold unique potential to manipulate excitons in TMDs and their heterostructures for application in optoelectronics and solar energy conversion.
Abstract: Light emission in two-dimensional (2D) transition metal dichalcogenides (TMDs) changes significantly with the number of layers and stacking sequence. While the electronic structure and optical absorption are well understood in 2D-TMDs, much less is known about exciton dynamics and radiative recombination. Here, we show first-principles calculations of intrinsic exciton radiative lifetimes at low temperature (4 K) and room temperature (300 K) in TMD monolayers with the chemical formula MX2 (X = Mo, W, and X = S, Se), as well as in bilayer and bulk MoS2 and in two MX2 heterobilayers. Our results elucidate the time scale and microscopic origin of light emission in TMDs. We find radiative lifetimes of a few picoseconds at low temperature and a few nanoseconds at room temperature in the monolayers and slower radiative recombination in bulk and bilayer than in monolayer MoS2. The MoS2/WS2 and MoSe2/WSe2 heterobilayers exhibit very long-lived (∼20–30 ns at room temperature) interlayer excitons constituted by ele...
575 citations
TL;DR: This study describes a new motif for designing highly efficient acceptors for organic solar cells that achieve an 8.3% power conversion efficiency in a solar cell, which is a record high for non-fullerene bulk heterojunctions.
Abstract: Despite numerous organic semiconducting materials synthesized for organic photovoltaics in the past decade, fullerenes are widely used as electron acceptors in highly efficient bulk-heterojunction solar cells. None of the non-fullerene bulk heterojunction solar cells have achieved efficiencies as high as fullerene-based solar cells. Design principles for fullerene-free acceptors remain unclear in the field. Here we report examples of helical molecular semiconductors as electron acceptors that are on par with fullerene derivatives in efficient solar cells. We achieved an 8.3% power conversion efficiency in a solar cell, which is a record high for non-fullerene bulk heterojunctions. Femtosecond transient absorption spectroscopy revealed both electron and hole transfer processes at the donor-acceptor interfaces. Atomic force microscopy reveals a mesh-like network of acceptors with pores that are tens of nanometres in diameter for efficient exciton separation and charge transport. This study describes a new motif for designing highly efficient acceptors for organic solar cells.
507 citations
TL;DR: It is shown that the light-matter interaction in monolayer WSe_{2} is strongly enhanced when the incoming electromagnetic wave is in resonance with the energy of the exciton states of strongly Coulomb bound electron-hole pairs below the electronic band gap.
Abstract: A three-order of magnitude increase in the optical second-harmonic emission of WSe${}_{2}$ monolayers is detected by tuning the exciting laser beam to the WSe${}_{2}$ exciton resonances
504 citations
TL;DR: In this paper, an ultrafast optical pump-probe was used to study the carrier recombination dynamics in MoS2 monolayers. But the probe was tuned to wavelengths much longer than the exciton line to make the probe transmission sensitive to the total population of photoexcited electrons and holes.
Abstract: In this Letter, we present nondegenerate ultrafast optical pump–probe studies of the carrier recombination dynamics in MoS2 monolayers. By tuning the probe to wavelengths much longer than the exciton line, we make the probe transmission sensitive to the total population of photoexcited electrons and holes. Our measurement reveals two distinct time scales over which the photoexcited electrons and holes recombine; a fast time scale that lasts ∼2 ps and a slow time scale that lasts longer than ∼100 ps. The temperature and the pump fluence dependence of the observed carrier dynamics are consistent with defect-assisted recombination as being the dominant mechanism for electron–hole recombination in which the electrons and holes are captured by defects via Auger processes. Strong Coulomb interactions in two-dimensional atomic materials, together with strong electron and hole correlations in two-dimensional metal dichalcogenides, make Auger processes particularly effective for carrier capture by defects. We pres...
478 citations
20 Apr 2015
TL;DR: In this article, the authors observed a single-photon emission from localized excitons in a monolayer of tungsten diselenide (WSe2), where the emitters appear at the edges of the flakes and are linearly polarized.
Abstract: Single-photon sources are basic building blocks for quantum communications, processing, and metrology. Solid-state quantum emitters in semiconductors have the potential for robust and reliable generation of photons, and atomically thin transition metal dichalcogenides, such as MoS2, MoSe2, WS2, and WSe2, are a promising new class of two-dimensional semiconductors with a direct optical bandgap in the visible or near-IR. Here, we observe bright and stable single-photon emission from localized excitons in a monolayer of tungsten diselenide (WSe2). The emitters appear at the edges of the flakes and are linearly polarized. The spectral width of their emission is below 120 μeV in a freestanding WSe2 monolayer. Photoluminescence excitation spectroscopy reveals the excitonic nature of the emitters and provides evidence that these single excitons originate from free excitons trapped in local potential wells at the edges of the atomically thin flakes. We find that the emitters can also be deterministically created by scratching the WSe2 monolayer. Their excellent spectral stability implies that these localized single-photon emitters could find application in optoelectronics. Our results light the way to single exciton physics and quantum optics with atomically thin semiconductors.
TL;DR: The key insights reported here—strong many-body effects and intrinsically rapid radiative recombination—are expected to be ubiquitous in atomically thin semiconductors.
Abstract: The band-edge optical response of transition metal dichalcogenides, an emerging class of atomically thin semiconductors, is dominated by tightly bound excitons localized at the corners of the Brillouin zone (valley excitons). A fundamental yet unknown property of valley excitons in these materials is the intrinsic homogeneous linewidth, which reflects irreversible quantum dissipation arising from system (exciton) and bath (vacuum and other quasiparticles) interactions and determines the timescale during which excitons can be coherently manipulated. Here we use optical two-dimensional Fourier transform spectroscopy to measure the exciton homogeneous linewidth in monolayer tungsten diselenide (WSe2). The homogeneous linewidth is found to be nearly two orders of magnitude narrower than the inhomogeneous width at low temperatures. We evaluate quantitatively the role of exciton-exciton and exciton-phonon interactions and population relaxation as linewidth broadening mechanisms. The key insights reported here—strong many-body effects and intrinsically rapid radiative recombination—are expected to be ubiquitous in atomically thin semiconductors.
TL;DR: The results pave the way for room-temperature polaritonic devices based on multiple-quantum-well van der Waals heterostructures, where polariton condensation and electrical polariton injection through the incorporation of graphene contacts may be realized.
Abstract: Layered materials can be assembled vertically to fabricate a new class of van der Waals heterostructures a few atomic layers thick, compatible with a wide range of substrates and optoelectronic device geometries, enabling new strategies for control of light-matter coupling. Here, we incorporate molybdenum diselenide/hexagonal boron nitride (MoSe2/hBN) quantum wells in a tunable optical microcavity. Part-light-part-matter polariton eigenstates are observed as a result of the strong coupling between MoSe2 excitons and cavity photons, evidenced from a clear anticrossing between the neutral exciton and the cavity modes with a splitting of 20 meV for a single MoSe2 monolayer, enhanced to 29 meV in MoSe2/hBN/MoSe2 double-quantum wells. The splitting at resonance provides an estimate of the exciton radiative lifetime of 0.4 ps. Our results pave the way for room-temperature polaritonic devices based on multiple-quantum-well van der Waals heterostructures, where polariton condensation and electrical polariton injection through the incorporation of graphene contacts may be realized.
TL;DR: This study reports a mechanism to electrically control second-order optical nonlinearities in monolayer WSe₂, an atomically thin semiconductor and paves the way towards a new platform for chip-scale, electrically tunable nonlinear optical devices based on two-dimensional semiconductors.
Abstract: Second-order optical nonlinearities can be controlled, up to room temperature, by electrostatic gating in a field-effect transistor made from atomically thin crystals of WSe2. Nonlinear optical frequency conversion, in which optical fields interact with a nonlinear medium to produce new field frequencies1, is ubiquitous in modern photonic systems. However, the nonlinear electric susceptibilities that give rise to such phenomena are often challenging to tune in a given material and, so far, dynamical control of optical nonlinearities remains confined to research laboratories as a spectroscopic tool2. Here, we report a mechanism to electrically control second-order optical nonlinearities in monolayer WSe2, an atomically thin semiconductor. We show that the intensity of second-harmonic generation at the A-exciton resonance is tunable by over an order of magnitude at low temperature and nearly a factor of four at room temperature through electrostatic doping in a field-effect transistor. Such tunability arises from the strong exciton charging effects in monolayer semiconductors3,4, which allow for exceptional control over the oscillator strengths at the exciton and trion resonances. The exciton-enhanced second-harmonic generation is counter-circularly polarized to the excitation laser due to the combination of the two-photon and one-photon valley selection rules5,6,7,8, which have opposite helicity in the monolayer. Our study paves the way towards a new platform for chip-scale, electrically tunable nonlinear optical devices based on two-dimensional semiconductors.
TL;DR: The experimental findings are consistent with theoretical predictions of spin-polarized conduction and valence bands at the K point of the Brillouin zone, with the minimum gap occurring between bands of opposite electron spin.
Abstract: Transition metal dichalcogenides in the class MX_{2} (M=Mo, W; X=S, Se) have been identified as direct-gap semiconductors in the monolayer limit. Here, we examine light emission of monolayer WSe_{2} using temperature-dependent photoluminescence and time-resolved photoluminescence spectroscopy. We present experimental evidence for the existence of an optically forbidden dark state of the band-gap exciton that lies tens of meV below the optically bright state. The presence of the dark state is manifest in the strong quenching of light emission observed at reduced temperatures. The experimental findings are consistent with theoretical predictions of spin-polarized conduction and valence bands at the K point of the Brillouin zone, with the minimum gap occurring between bands of opposite electron spin.
TL;DR: In this paper, transient absorption spectroscopy was used to show that the high PL QY can be attributed to negligible electron or hole trapping pathways in CsPbBr3 QDs: ∼94% of lowest excitonic states decayed with a singleexponential time constant of 4.5 ± 0.2 ns.
Abstract: Recently reported colloidal lead halide perovskite quantum dots (QDs) with tunable photoluminescence (PL) wavelengths covering the whole visible spectrum and exceptionally high PL quantum yields (QYs, 50-90%) constitute a new family of functional materials with potential applications in light-harvesting and -emitting devices. By transient absorption spectroscopy, we show that the high PL QYs (∼79%) can be attributed to negligible electron or hole trapping pathways in CsPbBr3 QDs: ∼94% of lowest excitonic states decayed with a single-exponential time constant of 4.5 ± 0.2 ns. Furthermore, excitons in CsPbBr3 QDs can be efficiently dissociated in the presence of electron or hole acceptors. The half-lives of electron transfer (ET) to benzoquinone and subsequent charge recombination are 65 ± 5 ps and 2.6 ± 0.4 ns, respectively. The half-lives for hole transfer (HT) to phenothiazine and the subsequent charge recombination are 49 ± 6 ps and 1.0 ± 0.2 ns, respectively. The lack of electron and hole traps and fast interfacial ET and HT rates are key properties that may enable the development of efficient lead halide perovskite QDs-based light-harvesting and -emitting devices.
TL;DR: It is demonstrated that that substrate-related effects such as screening and disorder in currently available specimens mask many anticipated physical phenomena and limit device applications of TMDCs.
Abstract: The optical response of semiconducting monolayer transition-metal dichalcogenides (TMDCs) is dominated by strongly bound excitons that are stable even at room temperature. However, substrate-related effects such as screening and disorder in currently available specimens mask many anticipated physical phenomena and limit device applications of TMDCs. Here, we demonstrate that that these undesirable effects are strongly suppressed in suspended devices. Extremely robust (photogain > 1,000) and fast (response time < 1 ms) photoresponse allow us to study, for the first time, the formation, binding energies, and dissociation mechanisms of excitons in TMDCs through photocurrent spectroscopy. By analyzing the spectral positions of peaks in the photocurrent and by comparing them with first-principles calculations, we obtain binding energies, band gaps and spin-orbit splitting in monolayer TMDCs. For monolayer MoS2, in particular, we obtain an extremely large binding energy for band-edge excitons, Ebind ≥ 570 meV. Along with band-edge excitons, we observe excitons associated with a van Hove singularity of rather unique nature. The analysis of the source-drain voltage dependence of photocurrent spectra reveals exciton dissociation and photoconversion mechanisms in TMDCs.
TL;DR: The exciton decay in monolayered WS2 exhibits a strong excitation density-dependence, which can be described using an exciton-exciton annihilation (two-particle Auger recombination) model, which is two orders of magnitude faster in the monolayer than in the bilayer and trilayer.
Abstract: We systematically investigate the exciton dynamics in monolayered, bilayered, and trilayered WS2 two-dimensional (2D) crystals by time-resolved photoluminescence (TRPL) spectroscopy. The exciton lifetime when free of exciton annihilation was determined to be 806 ± 37 ps, 401 ± 25 ps, and 332 ± 19 ps for WS2 monolayer, bilayer, and trilayer, respectively. By measuring the fluorescence quantum yields, we also establish the radiative and nonradiative lifetimes of the direct and indirect excitons. The exciton decay in monolayered WS2 exhibits a strong excitation density-dependence, which can be described using an exciton–exciton annihilation (two-particle Auger recombination) model. The exciton–exciton annihilation rate for monolayered, bilayered, and trilayered WS2 was determined to be 0.41 ± 0.02, (6.00 ± 1.09) × 10−3 and (1.88 ± 0.47) × 10−3 cm2 s−1, respectively. Notably, the exciton–exciton annihilation rate is two orders of magnitude faster in the monolayer than in the bilayer and trilayer. We attribute the much slower exciton–exciton annihilation rate in the bilayer and trilayer to reduced many-body interaction and phonon-assisted exciton–exciton annihilation of indirect excitons.
TL;DR: Here it is realized and confirmed that a synthetic superlattice of monolayer molybdenum disulphide forms an optoelectronic crystal capable of broadband light absorption and efficient funnelling of photogenerated excitons to points of maximum strain at the artificial-atom nuclei.
Abstract: The isolation of the two-dimensional semiconductor molybdenum disulphide introduced a new optically active material possessing a band gap that can be facilely tuned via elastic strain. As an atomically thin membrane with exceptional strength, monolayer molybdenum disulphide subjected to biaxial strain can embed wide band gap variations overlapping the visible light spectrum, with calculations showing the modified electronic potential emanating from point-induced tensile strain perturbations mimics the Coulomb potential in a mesoscopic atom. Here we realize and confirm this ‘artificial atom’ concept via capillary-pressureinduced nanoindentation of monolayer molybdenum disulphide from a tailored nanopattern, and demonstrate that a synthetic superlattice of these building blocks forms an optoelectronic crystal capable of broadband light absorption and efficient funnelling of photogenerated excitons to points of maximum strain at the artificial-atom nuclei. Such twodimensional semiconductors with spatially textured band gaps represent a new class of materials, which may find applications in next-generation optoelectronics or photovoltaics.
TL;DR: In this article, a fine excited state modulation was carried out to reach a golden combination of the high PL efficiency locally emissive (LE) component and the high exciton utilizing charge transfer (CT) component in one excited state.
Abstract: Excited state characters and components play a decisive role in photoluminescence (PL) and electroluminescence (EL) properties of organic light-emitting materials (OLEDS). Charge-transfer (CT) state is beneficial to enhance the singlet exciton utilizations in fluorescent OLEDs by an activated reverse intersystem crossing process, due to the minimized singlet and triplet energy splitting in CT excitons. However, the dominant CT component in the emissive state significantly reduces the PL efficiency in such materials. Here, the strategy is to carry out a fine excited state modulation, aiming to reach a golden combination of the high PL efficiency locally emissive (LE) component and the high exciton utilizing CT component in one excited state. As a result, a quasi-equivalent hybridization of LE and CT components is obtained in the emissive state upon the addition of only an extra phenyl ring in the newly synthesized material 4-[2-(4′-diphenylamino-biphenyl-4-yl)-phenanthro[9,10-d]imidazol-1-yl]-benzonitrile (TBPMCN), and the nondoped OLED of TBPMCN exhibited a record-setting performance: a pure blue emission with a Commission Internationale de L'Eclairage coordinate of (0.16, 0.16), a high external quantum efficiency of 7.8%, and a high yield of singlet exciton of 97% without delayed fluorescence phenomenon. The excited state modulation could be a practical way to design low-cost, high-efficiency fluorescent OLED materials.
TL;DR: Continuous tuning of the exciton binding energy in monolayer WS_{2} by means of an externally applied voltage in a field-effect transistor device is demonstrated and the renormalization of the quasiparticle band gap is tracked.
Abstract: We demonstrate continuous tuning of the exciton binding energy in monolayer WS_{2} by means of an externally applied voltage in a field-effect transistor device. Using optical spectroscopy, we monitor the ground and excited excitonic states as a function of gate voltage and track the evolution of the quasiparticle band gap. The observed decrease of the exciton binding energy over the range of about 100 meV, accompanied by the renormalization of the quasiparticle band gap, is associated with screening of the Coulomb interaction by the electrically injected free charge carriers at densities up to 8×10^{12} cm^{-2}. Complete ionization of the excitons due to the electrical doping is estimated to occur at a carrier density of several 10^{13} cm^{-2}.
TL;DR: The analysis of the measurements indicates that in complete analogy with other semiconducting transition metal dichalchogenides (TMDs) the dominant PL emission peaks originate from direct transitions associated with recombination of excitons and trions.
Abstract: We study the evolution of the band gap structure in few-layer MoTe2 crystals, by means of low-temperature microreflectance (MR) and temperature-dependent photoluminescence (PL) measurements. The analysis of the measurements indicate that in complete analogy with other semiconducting transition metal dichalchogenides (TMDs) the dominant PL emission peaks originate from direct transitions associated with recombination of excitons and trions. When we follow the evolution of the PL intensity as a function of layer thickness, however, we observe that MoTe2 behaves differently from other semiconducting TMDs investigated earlier. Specifically, the exciton PL yield (integrated PL intensity) is identical for mono and bilayer, decreases slightly for trilayer, and it is significantly lower in the tetralayer. The analysis of this behavior and of all our experimental observations is fully consistent with mono and bilayer MoTe2 being direct band gap semiconductors with tetralayer MoTe2 being an indirect gap semiconduct...
TL;DR: These states are identified in monolayers of MoS2 and WS2 supported on fused silica by means of photoluminescence excitation spectroscopy and imply an exciton binding energy of 0.32 eV.
Abstract: We have identified excited exciton states in monolayers of MoS2 and WS2 supported on fused silica by means of photoluminescence excitation spectroscopy. In monolayer WS2, the positions of the excited A exciton states imply an exciton binding energy of 0.32 eV. In monolayer MoS2, excited exciton transitions are observed at energies of 2.24 and 2.34 eV. Assigning these states to the B exciton Rydberg series yields an exciton binding energy of 0.44 eV.
TL;DR: In this article, the 1s-2p resonance of 1s A excitons in a single-layer WSe2 was analyzed and the decay dynamics of small-momentum exciton decay was shown to be ultrafast.
Abstract: Atomically thin two-dimensional crystals have revolutionized materials science. In particular, monolayer transition metal dichalcogenides promise novel optoelectronic applications, owing to their direct energy gaps in the optical range. Their electronic and optical properties are dominated by Coulomb-bound electron-hole pairs called excitons, whose unusual internal structure, symmetry, many-body effects and dynamics have been vividly discussed. Here we report the first direct experimental access to all 1s A excitons, regardless of momentum--inside and outside the radiative cone--in single-layer WSe2. Phase-locked mid-infrared pulses reveal the internal orbital 1s-2p resonance, which is highly sensitive to the shape of the excitonic envelope functions and provides accurate transition energies, oscillator strengths, densities and linewidths. Remarkably, the observed decay dynamics indicates an ultrafast radiative annihilation of small-momentum excitons within 150 fs, whereas Auger recombination prevails for optically dark states. The results provide a comprehensive view of excitons and introduce a new degree of freedom for quantum control, optoelectronics and valleytronics of dichalcogenide monolayers.
TL;DR: In this article, the authors demonstrate that 2D MoS2/WS2 heterostructures can enable equally efficient interlayer exciton relaxation regardless the epitaxy and orientation of the stacking.
Abstract: Semiconductor heterostructures provide a powerful platform to engineer the dynamics of excitons for fundamental and applied interests. However, the functionality of conventional semiconductor heterostructures is often limited by inefficient charge transfer across interfaces due to the interfacial imperfection caused by lattice mismatch. Here we demonstrate that MoS2/WS2 heterostructures consisting of monolayer MoS2 and WS2 stacked in the vertical direction can enable equally efficient interlayer exciton relaxation regardless the epitaxy and orientation of the stacking. This is manifested by a similar 2 orders of magnitude decrease of photoluminescence intensity in both epitaxial and nonepitaxial MoS2/WS2 heterostructures. Both heterostructures also show similarly improved absorption beyond the simple superimposition of the absorptions of monolayer MoS2 and WS2. Our result indicates that 2D heterostructures bear significant implications for the development of photonic devices, in particular those requestin...
TL;DR: The structural, morphological and optical properties of AB(Br(1-x)Cl(x))3 (where, A = CH3NH3(+), B = Pb(2+) and x = 0 to 1) perovskite semiconductor are reported and it is shown that the lattice constant varies linearly with the fractional chlorine content satisfying Vegards law.
Abstract: We report on the structural, morphological and optical properties of AB(Br1–xClx)3 (where, A = CH3NH3+, B = Pb2+ and x = 0 to 1) perovskite semiconductor and their successful demonstration in green and blue emissive perovskite light emitting diodes at room temperature. The bandgap of perovskite thin film is tuned from 2.42 to 3.16 eV. The onset of optical absorption is dominated by excitonic effects. The coulomb field of the exciton influences the absorption at the band edge. Hence, it is necessary to explicitly account for the enhancement of the absorption through the Sommerfield factor. This enables us to correctly extract the exciton binding energy and the electronic bandgap. We also show that the lattice constant varies linearly with the fractional chlorine content satisfying Vegards law.
TL;DR: Yang et al. as discussed by the authors used phase-shifting interferometry to determine the number of phosphorene layers, and confirmed the results with reliable photoluminescence measurements.
Abstract: Monolayer phosphorene provides a unique two-dimensional (2D) platform to investigate the fundamental dynamics of excitons and trions (charged excitons) in reduced dimensions. However, owing to its high instability, unambiguous identification of monolayer phosphorene has been elusive. Consequently, many important fundamental properties, such as exciton dynamics, remain underexplored. We report a rapid, noninvasive, and highly accurate approach based on optical interferometry to determine the layer number of phosphorene, and confirm the results with reliable photoluminescence measurements. Furthermore, we successfully probed the dynamics of excitons and trions in monolayer phosphorene by controlling the photo-carrier injection in a relatively low excitation power range. Based on our measured optical gap and the previously measured electronic energy gap, we determined the exciton binding energy to be ∼0.3 eV for the monolayer phosphorene on SiO2/Si substrate, which agrees well with theoretical predictions. A huge trion binding energy of ∼100 meV was first observed in monolayer phosphorene, which is around five times higher than that in transition metal dichalcogenide (TMD) monolayer semiconductor, such as MoS2. The carrier lifetime of exciton emission in monolayer phosphorene was measured to be ∼220 ps, which is comparable to those in other 2D TMD semiconductors. Our results open new avenues for exploring fundamental phenomena and novel optoelectronic applications using monolayer phosphorene. An optical scheme for determining the number of monolayers on two-dimensional materials has been developed. Research into two-dimensional materials is thriving, but developing a way to identify a single monolayer has proved challenging. Now, Jiong Yang and co-workers have used phase-shifting interferometry to deduce the number of phosphorene layers. They then performed power-dependent photoluminescence measurements to determine various excitonic properties of a monolayer on a silicon oxide/silicon substrate. They obtained an exciton binding energy of about 0.3 electron volts, which agrees well with theoretical predictions. The researchers measured a carrier lifetime of approximately 220 picoseconds, which is comparable to that of the transition-metal dichalcogenides, another class of two-dimensional semiconductors. They also measured a trion binding energy of about 100 milli-electron volts, which is around five times higher than that of transition-metal dichalcogenides.
TL;DR: Using a 1D model system, it is demonstrated that exciton conductance in organic materials can be enhanced by several orders of magnitude when the molecules are strongly coupled to an electromagnetic mode.
Abstract: We demonstrate that exciton conductance in organic materials can be enhanced by several orders of magnitude when the molecules are strongly coupled to an electromagnetic mode. Using a 1D model system, we show how the formation of a collective polaritonic mode allows excitons to bypass the disordered array of molecules and jump directly from one end of the structure to the other. This finding could have important implications in the fields of exciton transistors, heat transport, photosynthesis, and biological systems in which exciton transport plays a key role.
TL;DR: In this article, optical spectroscopy studies of thin layers of the transition metal dichalcogenide WSe2, with thickness ranging from mono-to tetra-layer and in the bulk limit, are presented.
Abstract: We present optical spectroscopy (photoluminescence and reflectance) studies of thin layers of the transition metal dichalcogenide WSe2, with thickness ranging from mono- to tetra-layer and in the bulk limit. The investigated spectra show the evolution of excitonic resonances as a function of layer thickness, due to changes in the band structure and, importantly, due to modifications of the strength of Coulomb interactions as well. The observed temperature-activated energy shift and broadening of the fundamental direct exciton are well accounted for by standard formalisms used for conventional semiconductors. A large increase of the photoluminescence yield with temperature is observed in a WSe2 monolayer, indicating the existence of competing radiative channels. The observation of absorption-type resonances due to both neutral and charged excitons in the WSe2 monolayer is reported and the effect of the transfer of oscillator strength from charged to neutral excitons upon an increase of temperature is demonstrated.
TL;DR: The finding of electrically induced robust emission opens up a possibility to boost the luminous efficiency of emerging 1L TMD light emitting diodes and indicates the strong Coulomb interactions in such a 2D material.
Abstract: Two-dimensional (2D) semiconductors, such as transition-metal dichalcogenide monolayers (TMD 1Ls), have attracted increasing attention owing to the underlying fundamental physics (e.g., many body effects) and the promising optoelectronic applications such as light-emitting diodes. Though much progress has been made, intrinsic excitonic states of TMD 1Ls are still highly debated in theory, which thirsts for direct experimental determination. Here, we report unconventional emission and excitonic fine structure in 1L WS2 revealed by electrical doping and photoexcitation, which reflects the interplay of exciton, trion, and other excitonic states. Tunable excitonic emission has been realized in a controllable manner via electrical and/or optical injection of charge carriers. Remarkably enough, the superlinear (i.e., quadratic) emission is unambiguously observed which is attributed to biexciton states, indicating the strong Coulomb interactions in such a 2D material. In a nearly neutral 1L WS2, trions and biexc...