Formation Of Charged Excitons Research Articles

Persistent Charging of CsPbBr3 Perovskite Nanocrystals Confined in Glass Matrix.

Manipulation of persistent charges in semiconductor nanostructure is the key point to obtain quantum bits towards the application of quantum memory and information devices. However, realizing persistent charge storage in semiconductor nano-systems is still very challenge due to the disturbance from crystal defects and environment conditions. Herein, the two-photon persistent charging induced long-lasting afterglow and charged exciton formation are observed in CsPbBr3 perovskite nanocrystals (NCs) confined in glass host with effective lifetime surpassing one second, where the glass inclosure provides effective protection. A method combining the femtosecond and second time-resolved transient absorption spectroscopy is explored to determine the persistent charging possibility of perovskite NCs unambiguously. Meanwhile, with temperature-dependent spectroscopy, the underlying mechanism of this persistent charging is elucidated. A two-channel carrier transfer model is proposed involving athermal quantum tunneling and slower thermal-assisted channel. On this basis, two different information storage devices are demonstrated with the memory time exceeding two hours under low-temperature condition. These results provide a new strategy to realize persistent charging in perovskite NCs and deepen the understanding of the underlying carrier kinetics, which may pave an alternative way towards novel information memory and optical data storage applications.

Large Biaxial Compressive Strain Tuning of Neutral and Charged Excitons in Single-Layer Transition Metal Dichalcogenides.

The absorption and emission of light in single-layer transition metal dichalcogenides are governed by the formation of excitonic quasiparticles. Strain provides a powerful technique to tune the optoelectronic properties of two-dimensional materials and thus to adjust their exciton energies. The effects of large compressive strain in the optical spectrum of two-dimensional (2D) semiconductors remain rather unexplored compared to those of tensile strain, mainly due to experimental constraints. Here, we induced large, uniform, biaxial compressive strain (∼1.2%) by cooling, down to 10 K, single-layer WS2, MoS2, WSe2, and MoSe2 deposited on polycarbonate substrates. We observed a significant strain-induced modulation of neutral exciton energies, with blue shifts up to 160 meV, larger than in any previous experiments. Our results indicate a remarkably efficient transfer of compressive strain, demonstrated by gauge factor values exceeding previous results and approaching theoretical expectations. At low temperatures, we investigated the effect of compressive strain on the resonances associated with the formation of charged excitons. In WS2, a notable reduction of gauge factors for charged compared to neutral excitons suggests an increase in their binding energy, which likely results from the effects of strain added to the influence of the polymeric substrate.

Charged Exciton Formation in Compact Polycrystalline Perovskite Thin Films

Charged Exciton Formation in Compact Polycrystalline Perovskite Thin Films

Many-Body Exciton and Intervalley Correlations in Heavily Electron-Doped WSe2 Monolayers

In monolayer transition-metal dichalcogenide semiconductors, many-body correlations can manifest in optical spectra when electron-hole pairs (excitons) are photoexcited into a 2D Fermi sea of mobile carriers. At low carrier densities, the formation of charged excitons (X±) is well documented. However, in WSe2 monolayers, an additional absorption resonance, often called X-', emerges at high electron density. Its origin is not understood. Here, we investigate the X-' state via polarized absorption spectroscopy of gated WSe2 monolayers in magnetic fields to 60T. Field-induced filling and emptying of the lowest optically active Landau level in the K' valley causes repeated quenching of the corresponding optical absorption. Surprisingly, these quenchings are accompanied by absorption changes to higher Landau levels in both K' and K valleys, which are unoccupied. These results cannot be reconciled within a single-particle picture, and demonstrate the many-body nature and intervalley correlations of the X-' quasiparticle state.

Strong photocharging effect in CsPbBr3 nanocrystal films

In this study, a strong photocharging effect has been detected by a spectroscopic study of colloidal CsPbBr3 quantum dot film. Photocharging induces the formation of charged excitons, even with the application of a very low excitation laser power. Thus, it can introduce a drastic problem on the optical properties of quantum dots when the charges are involved. The charge exciton then can absorb another photon from the next laser pulse, leading to a trion state. The presence of the trion state was proven by studying its behavior against three effects, namely laser power, electric field (EF) and temperature (T) dependence. The trion area of the peak increases linearly with the increasing of the laser power, which we consider to be an indication of the influence of a strong photocharging effect, suggesting that long-lived charges can be kept trapped in quantum dots (QDs). It was also found the trion is affected by an external electric field, due to the weak Stark effect. It is more sensitive than the excitonic state to the electric field due to the presence of electron–electron repulsion forces. A red-shift behavior was observed when heating the sample above 180 K, which is a strong evidence of the presence of a trapping state.

Electrical control of single-photon emission in highly charged individual colloidal quantum dots.

Electron transfer to an individual quantum dot promotes the formation of charged excitons with enhanced recombination pathways and reduced lifetimes. Excitons with only one or two extra charges have been observed and exploited for very efficient lasing or single-quantum dot light-emitting diodes. Here, by room-temperature time-resolved experiments on individual giant-shell CdSe/CdS quantum dots, we show the electrochemical formation of highly charged excitons containing more than 12 electrons and 1 hole. We report the control over intensity blinking, along with a deterministic manipulation of quantum dot photodynamics, with an observed 210-fold increase in the decay rate, accompanied by 12-fold decrease in the emission intensity, while preserving single-photon emission characteristics. These results pave the way for deterministic control over the charge state, and room-temperature decay rate engineering for colloidal quantum dot-based classical and quantum communication technologies.

Exciton center-of-mass localization and dielectric environment effect in monolayer WS2

The ultrathin transition metal dichalcogenides (TMDs) have emerged as promising materials for various applications using two dimensional semiconductors. They have attracted increasing attention due to their unique optical properties originate from neutral and charged excitons. In this paper, we study the strong localization of exciton center-of-mass motion within random potential fluctuations caused by the monolayer defects. Here, we report negatively charged exciton formation in monolayer TMDs, notably tungsten disulfide WS2. Our theory is based on an effective mass model of neutral and charged excitons, parameterized by ab-initio calculations. Taking into the account the strong correlation between the monolayer WS2 and the surrounding dielectric environment, our theoretical results are in good agreement with one-photon photoluminescence (PL) and reflectivity measurements. We also show that the exciton state with p-symmetry, experimentally observed by two-photon PL emission, is energetically below the 2s-state. We use the equilibrium mass action law, to quantify the relative weight of exciton and trion PL. We show that exciton and trion emission can be tuned and controlled by external parameters like temperature, pumping, and injection electrons. Finally, in comparison with experimental measurements, we show that exciton emission in monolayer tungsten dichalcogenides is substantially reduced. This feature suggests that free exciton can be trapped in disordered potential wells to form a localized exciton and therefore offers a route toward novel optical properties.

Droplet etched GaAs quantum dots close to surfaces and metallic interfaces

GaAs quantum dots (QDs) with a thin cap layer are studied as building blocks for self-aligned hybrids with a metallic nanostructure (MN). Both constituents are filled into a nanohole template that is drilled into an AlGaAs surface by self-assembled local droplet etching during molecular beam epitaxy. In a first series of samples, the interaction of a near AlGaAs surface with a single QD at varied distance is studied using microphotoluminescence (PL) spectroscopy. With decreasing distance down to 12.5 nm, surface charges cause an increase in the exciton radiative lifetime, the formation of charged excitons, and a broadening of the exciton PL peaks. The PL peak broadening is quantitatively analyzed on the basis of an analytical model assuming temporal fluctuations of the surface charge. In a second sample series, the nanoholes are filled in addition with an Au nanostructure. The optical spectra are similar to those from QDs without a metal but with a slightly stronger PL peak broadening. For a small distance of 12.5 nm clearly within the optical near-field of the MN, the QDs show a typical PL linewidth of 430 μeV that is still small enough to separate different excitonic lines.

(Invited) Transition Metal Dichalcogenide Monolayers As Quantum Wells

Recently, transition metal dichalcogenide (TMDC) monolayers, such as molybdenum disulfide (MoS2) and tungsten diselenide (WSe2), have attracted strong attention as novel two-dimensional (2D) semiconducting materials due to their large bandgap (1−2 eV) and excellent transport properties. Moreover, the thickness of monolayer TMDCs is less than 1 nm, which is one of the thinnest materials, and it leads to strong confinement effects, resulting in large binding energy of exciton (> 100 meV) and formation of charged excitons. Particularly, due to their layered structure, there are no dangling-bond states on the surface of TMDC monolayers and it could be an ideal quantum well. Here, we will report the optical and thermoelectric properties of TMDC monolayers as quantum wells. In this study, we fabricated ion-gel-gated EDLTs using large-area TMDC monolayers, MoS2 and WSe2, grown by chemical vapor deposition [1-8]. The Fermi level of TMDCs can be continuously shifted by applying gate voltage, and we can induce both hole and electron transport in these devices. The hole mobility of WSe2 can be enhanced up to 90 cm2/Vs at high carrier density of 1014 cm-2, whereas the MoS2 showed electron mobility of 60 cm2/Vs. Using EDLT technique, we investigated the electric field modulation spectroscopy of TMDC monolayers and the clear signature of quantum well, such as quantum confined Stark effect, was observed. Very importantly, it is well-clarified that, in quantum wells, the bandgap reduction by electric field is simply proportional to width of quantum well. Therefore, in ultimately thin TMDC monolayers, we need extremely strong electric field to realize obvious spectral modulation, which is very difficult for conventional field-effect transistors with oxide dielectric layers such as SiO2 and Al2O3. To break this limitation, we used EDLT technique and firstly observed the obvious evidence of quantum confined Stark effect. Seebeck coefficient, which is the proportional constant to the voltage generation against induced temperature gradient, is a significant factor to design thermoelectric materials. Importantly, according to the Mott equation, Seebeck coefficient is proportional to the energy derivative of the electronic density of states at around Fermi energy and, therefore, low-dimensional structures/materials are suitable for thermoelectric applications. We also investigated Fermi energy dependence of Seebeck coefficient in TMDC monolayers using EDLT technique. As the results, we observed clear enhancement in thermoelectric properties due to the low-dimensional effect. [1] J. Pu, L.-J. Li, T. Takenobu, et al., Nano Lett. 12, 4013 (2012). [2] J.-K. Huang, T. Takenobu, L.-J. Li, et al., ACS Nano. 8, 923 (2014). [3] J. Pu, L.-J. Li, T. Takenobu, et al., Appl. Phys. Lett., 103, 23505 (2013). [4] J. Pu, L.-J. Li and T. Takenobu, Phys. Chem. Chem. Phys., 10.1039/C3CP55270E (2014). [5] Y.-H. Chang, T. Takenobu, L.-J. Li, et al., ACS Nano. 8, 8582 (2014). [6] C.-H. Chen, T. Takenobu, L.-J. Li, et al., 2D Materials, 1, 034001 (2014). [7] L. Chu, T. Takenobu, G. Eda , et al., Scientific Reports, 4, 7293 (2014) [8] K. Funahashi, L.-J. Li, T. Takenobu, et al., Jpn. J. Appl. Phys. 54, 06FF06 (2015).

Large-area single-layer MoSe2 and its van der Waals heterostructures.

Layered structures of transition metal dichalcogenides stacked by van der Waals interactions are now attracting the attention of many researchers because they have fascinating electronic, optical, thermoelectric, and catalytic properties emerging at the monolayer limit. However, the commonly used methods for preparing monolayers have limitations of low yield and poor extendibility into large-area applications. Herein, we demonstrate the synthesis of large-area MoSe2 with high quality and uniformity by selenization of MoO3 via chemical vapor deposition on arbitrary substrates such as SiO2 and sapphire. The resultant monolayer was intrinsically doped, as evidenced by the formation of charged excitons under low-temperature photoluminescence analysis. A van der Waals heterostructure of MoSe2 on graphene was also demonstrated. Interestingly, the MoSe2/graphene heterostructures show strong quenching of the characteristic photoluminescence from MoSe2, indicating the rapid transfer of photogenerated charge carriers between MoSe2 and graphene. The development of highly controlled heterostructures of two-dimensional materials will further promote advances in the physics and chemistry of reduced dimensional systems and will provide novel applications in electronics and optoelectronics.

Carrier-Collision-Induced Formation of Charged Excitons and Ultrafast Dynamics Fluorescence Spectra

After a hole injection layer is inserted into a polymer light-emitting material, the injection of positive charge not only easily causes distortion in the conjugated polymer chain but also produces positive polarons. The ultrafast dynamics shows that, when the positive polaron approaches and collides with the triplet exciton, that exciton will become charged, whereby the non-emissive triplet exciton becomes radiative and emits light. Furthermore, the lifetime of the charged triplet exciton is longer than the singlet exciton. This paper explicitly depicts the dynamic fluorescence spectra of the radiative transition of the charged triplet exciton occurring during the decay of the charged exciton, and also exhibits the difference between traditional adiabatic dynamics and non-adiabatic dynamics.

Dynamics of Trion Formation inInxGa1−xAsQuantum Wells

We show a double path mechanism for the formation of charged excitons (trions); they are formed through bi- and trimolecular processes. This directly implies that both negatively and positively charged excitons coexist in a quantum well, even in the absence of excess carriers. The model is substantiated by time-resolved photoluminescence experiments performed on a very high quality InxGa1-xAs quantum well sample, in which the photoluminescence contributions at the energy of the trion and exciton and at the band edge can be clearly separated and traced over a broad range of times and densities. The unresolved discrepancy between the theoretical and experimental radiative decay time of the exciton in a doped semiconductor quantum well is explained by the same model.

Optically injected electron gas and formation of charged excitons in type-II ZnSe/BeTe quantum wells

We have investigated the effect of accumulation of photo-created electrons in ZnSe/BeTe type-II quantum well (QW) structures by means of a pump-and-probe method in high magnetic field up to 7 T. A cw-diode laser was used as a pumping light source. With increasing pumping intensity neutral exciton (X) absorption are weakened and a distinct peak structure due to negatively charged exciton (X−) was observed at a 6 meV lower energy side of the X in ZnSe QW layers. The magnetic field dependence of circular polarization of X− contains information about the density of twodimensional electron gas (2DEG). We found that high-density 2DEG (∼1×1011cm−2) is optically injected by successfully using type-II quantum structures without n-doped layers.

Photoluminescence ofp-doped quantum wells with strong spin splitting

The spectroscopic properties of a spin polarized two-dimensional hole gas are studied in modulation doped (Cd,Mn)Te quantum wells. The giant Zeeman effect induces a significant spin splitting even at very small values of the applied field. Several methods of measuring the carrier density (Hall effect, filling factors of the Landau levels at high field, various manifestations of Moss-Burstein shifts) are described and calibrated. The value of the spin splitting needed to fully polarize the hole gas, evidences a strong enhancement of the spin susceptibility of the hole gas due to carrier-carrier interaction. At small values of the spin splitting, whatever the carrier density (non zero) is, photoluminescence lines are due to the formation of charged excitons in the singlet state. Spectral shifts in photoluminescence and in transmission (including an "excitonic Moss-Bustein shift") are observed and discussed in terms of excitations of the partially or fully polarized hole gas. At large spin splitting, and without changing the carrier density, the singlet state of the charged exciton is destabilized in favour of a triplet state configuration of holes. The binding energy of the singlet state is thus measured and found to be independent of the carrier density (in contrast with the splitting between the charged exciton and the neutral exciton lines). The state stable at large spin splitting is close to the neutral exciton at low carrier density, and close to an uncorrelated electron-hole pair at the largest values of the carrier density achieved. The triplet state gives rise to a characteristic double-line structure with an indirect transition to the ground state (with a strong phonon replica) and a direct transition to an excited state of the hole gas.

The influence of carrier diffusion on the formation of charged excitons in InAs/GaAs quantum dots

It is demonstrated that the photoluminescence spectra of single self-assembled quantum dots are very sensitive to the excitation energy and crystal temperature. A qualitative explanation is given in terms of the effective diffusivity of the photogenerated carriers, determined by the experimental conditions, which influence the capture probability and hence also the charge state of the quantum dots (QDs). This is proposed as an effective tool to populate quantum dots with extra electrons in order to study phenomena involving charged excitons.

Carrier Diffusion in the Barrier Enabling Formation of Charged Excitons in InAs/GaAs Quantum Dots

It is demonstrated that the photoluminescence spectra of single self-assembled quantum dots are very sensitive to the experimental conditions, such as excitation energy and crystal temperature. A q ...

Influence of excitation energy on charged exciton formation in self-assembled InAs single quantum dots

Influence of excitation energy on charged exciton formation in self-assembled InAs single quantum dots

Magneto-optics of charged excitons in ZnSe/ZnMgSSe quantum wells

We report magneto-optical studies of ZnSe/(Zn,Mg)(S,Se) quantum wells with n-type modulation doping. Polarized photoluminescence and spin-flip Raman scattering spectroscopy is used to investigate the energy and spin structure of neutral and negatively charged excitons. The binding energy of charged excitons changes very little in magnetic fields up to 15 T. An inverted population of neutral exciton spin sublevels is found in magnetic fields; it is associated with the spin-dependent formation of charged excitons.

Spin Coherence and Formation Dynamics of Charged Excitons in CdTe/CdMgZnTe Quantum Well

We report on a detailed study of the dynamics of charged and neutral excitons in a p-doped CdTe/CdMgZnTe quantum well by time-resolved photoluminescence under strictly resonant excitation. The 1.5 ps resolution of the experimental set-up allows us to resolve the rise time of the charged exciton photoluminescence after resonant excitation of the neutral exciton. We measure τrise ≈ 13 ps. A fit of the photoluminescence dynamics, which takes into account the different relaxation mechanisms of the charged and neutral excitons, yields the charged exciton formation time. We find 65 ps. In a second part the spin coherence dynamics of the charged and neutral excitons in a transverse applied magnetic field is studied. We show that neither the spin orientation nor the spin coherence of electrons is destroyed by the formation process of charged excitons via exciton states.

Negatively charged exciton formation in an asymmetric double CdTe/(Cd,Mn)Te QWs

An optical spectroscopy study of unintentionally doped CdTe/Cd 0.88Mn 0.12Te double quantum wells (QWs) revealed formation of a negatively charged exciton state in the thicker QW due to the interwell tunneling of photoexcited electrons. A suggested model based on the chemical equilibrium requirement has explained the behavior of the X −-intensity in the photoluminescence spectra with excitation energy and excitation density variation. In-plane concentration of X − states at constant excitation has been estimated to be limited by 6×10 10 cm −2. X − binding energy is found to be 3.7 meV.