Photogenerated Carrier Density Research Articles

V2O5 nanoflakes for broad-spectral-response self-powered photodetectors with a high on/off ratio and high detectivity

In this study, V2O5 nanoflakes (NFs) was coated on Si substrate by DC sputtering to obtain V2O5 NFs/n-Si heterojunction. To utilize V2O5 NFs as a broadband photodetector, absorbance spectra were studied using UV−Vis−near-IR spectroscopy. Cut-off wavelength was 530 nm. Furthermore, energy dispersive x-ray, x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and scanning electron microscope analyses of V2O5 NFs were achieved. The V2O5 NFs/n-Si device gave a very high rectifying ratio of 1.18 × 104 in the dark and at zero bias, it has self-powered mode and an on/off ratio of 1.29 × 106. Optical analyses of the V2O5 NFs/n-Si heterojunction device were studied in detail under UV (365, 395 nm) and IR (850 nm) illumination as well as visible light with varying light intensities. Analysis of experimental studies showed that the device has a high photoresponse under all illuminations. For optical analysis based on I–V measurements, responsivity, detectivity, on/off ratio, external quantum efficiency (EQE), normalized photocurrent-dark-current ratio and noise-equivalent power (NEP) analyses were achieved. The maximum values of responsivity from measurements under visible, UV (395 nm) and IR illumination (850 nm) were 104, 882 and 850 mA W−1 for −2.0 V, respectively. Detectivity values are maximized at V = 0 V and are 6.84 × 1011, 7.87 × 1012 and 6.87 × 1012Jones for the same illuminations respectively. With increasing intensity, the rectification ratio and NEP decreased while the other parameters generally increased. The increase in performance at increasing visible intensity was attributed to the increase in photogenerated carrier density at high intensities, and the high performance in the UV region was attributed to the high light absorption of V2O5 NFs in the UV region.

One-step synthesis of 2D/2D gC3N4/MoS2 composites for effective photocatalytic hydrogen evolution

Two-dimensional (2D) heterostructures have proven their efficiency in separating charges. Herein, a gC3N4/MoS2 (CN/MoS2) photocatalyst, in the form of 2D/2D van der Waals heterojunction with a type-II band alignment, was synthesized using a novel one-step method. The resulting in-situ formed heterojunction interface enhances electron transfer from gC3N4 to MoS2. The CN/MoS2 photocatalyst exhibits exceptional activity in generating hydrogen from water under simulated solar light. Its hydrogen production rate was 1285 μmol/h/g, which was approximately 3 times higher compared to gC3N4. This superiority is due to a significant increase in photogenerated carrier density and efficient charge separation facilitated by the type-II heterojunction. This study demonstrates a straightforward one-step approach for synthesizing 2D/2D composite photocatalysts, highlighting the effective synergy between different 2D materials in promoting hydrogen generation.

Resonant Tunneling-Enhanced Photoresponsivity in a Twisted Graphene van der Waals Heterostructure.

Leveraging its ultrahigh carrier mobility, zero-bandgap linear dispersion, and extremely short response time, graphene exhibits remarkable potential in ultrafast broad-band photodetection. Nonetheless, the inherently low responsivity of graphene photodetectors, due to the low photogenerated carrier density, significantly impedes the development of practical devices. In this study, we present an improved photoresponse within a graphene-hexagonal boron nitride-graphene vertical tunnel junction device, where the crystallographic orientation of the two graphene electrodes is aligned. Through meticulous device structure design and the adjustment of bias and gate voltages, we observe a 2 orders of magnitude increase in tunneling photocurrent, which is attributed to the momentum-conserving resonant electron tunneling. The enhanced external photoresponsivity is evident across a wide temperature and spectral range and achieves 0.7 A/W for visible light excitation. This characteristic, coupled with the device's negative differential conductance, suggests a novel avenue for highly efficient photodetection and high-frequency, logic-based optoelectronics using van der Waals heterostructures.

Localisation-to-delocalisation transition of moiré excitons in WSe2/MoSe2 heterostructures.

Moiré excitons (MXs) are electron-hole pairs localised by the periodic (moiré) potential forming in two-dimensional heterostructures (HSs). MXs can be exploited, e.g., for creating nanoscale-ordered quantum emitters and achieving or probing strongly correlated electronic phases at relatively high temperatures. Here, we studied the exciton properties of WSe2/MoSe2 HSs from T = 6 K to room temperature using time-resolved and continuous-wave micro-photoluminescence also under a magnetic field. The exciton dynamics and emission lineshape evolution with temperature show clear signatures that MXs de-trap from the moiré potential and turn into free interlayer excitons (IXs) for temperaturesabove 100 K. The MX-to-IX transition is also apparent from the exciton magnetic moment reversing its sign when the moiré potential is not capable of localising excitons at elevated temperatures. Concomitantly, the exciton formation and decay times reduce drastically. Thus, our findings establish the conditions for a truly confined nature of the exciton states in a moiré superlattice with increasing temperature and photo-generated carrier density.

Interface passivation with Ti3C2Tx-MXene doped PMMA film for highly efficient and stable inverted perovskite solar cells

PMMA:MXene passivates a perovskite/ETL interface boosting performance and stability. Less interfacial defects increase the charge lifetime resulting in a higher density of photo-generated carriers.

Understanding the Heterointerfaces in Perovskite Solar Cells via Hole Selective Layer Surface Functionalization.

Interfaces in perovskite solar cells (PSCs) play a pivotal role in determining device performance by influencing charge transport and recombination. Understanding the physical processes at these interfaces is essential for achieving high-power conversion efficiency in PSCs. Particularly, the interfaces involving oxide-based transport layers are susceptible to defects like dangling bonds, excess oxygen, or oxygen deficiency. To address this issue, the surface of NiOx is passivated using octadecylphosphonic acid (ODPA), resulting in improved charge transport across the perovskite hole transport layer (HTL) interface. This surface treatment has led to the development of hysteresis-free devices with an impressive ≈13% increase in power conversion efficiency. Computational studies have explored the halide perovskite architecture of ODPA-treated HTL/Perovskite, aiming to unlock superior photovoltaic performance. The ODPA surface functionalization has demonstrated enhanced device performance, characterized by superior charge exchange capacity. Moreover, higher band-to-band recombination in photoluminescence and electroluminescence indicates presence of lower mid-gap energy states, thereby increasing the effective photogenerated carrier density. These findings are expected to promote the utilization of various phosphonic acid-based self-assembly monolayers for surface passivation of oxide-based transport layers in perovskite solar cells. Ultimately, this research contributes to the realization of efficient halide PSCs by harnessing the favorable architecture of NiOx interfaces.

Enhanced photocatalytic reduction of CO2 using a novel 2D/0D SnS2/CeO2 binary photocatalyst with Z-scheme heterojunction and oxygen vacancy

In this work, suitable band gap width, strong photo-response to visible light, and a high photogenerated carrier density offered by an important member of layered sulfides, SnS2, were kept in view for designing and producing a highly effective photocatalyst for CO2 reduction. Hence, SnS2/CeO2 heterojunction composite photocatalysts were triumphally fabricated by a facile in situ hydrothermal synthesis method, combining advantages of the built-in electric field and oxygen vacancies of two materials. Various characterization techniques were exploited to scrutinize the photocatalyst’s structure and confirm that the carrier transport efficiency and photocatalytic performance are enhanced by the synergistic impact of the heterojunction structure and presence of oxygen vacancies. Owing to the successful loading of zero-dimensional CeO2 nanoparticles onto two-dimensional SnS2 nanosheets, the 2D/0D structure improves the photo-response range of the composites and provides faster electron transfer rates. The composite material maintained the hydrophobic characteristic of single SnS2, while inhibiting the hydrophilic property of single CeO2. As a result, CO and CH4 yields of prepared binary Z-type heterojunction photocatalysts are 24.6 and 4.4 folds higher than those of single SnS2, respectively. This innovative study will contribute to exploration of SnS2-based photocatalytic materials and provide a new feasible direction for light energy-driven CO2 reduction.

Dopant-driven recombination delay and ROS enhancement in nanoporous Cd1-xCuxS heterogeneous photocatalyst for the degradation of DR-23 dye under visible light irradiation

Developing an efficient heterogeneous photocatalyst for environmental remediation and treatment strategies using visible light harvesting processes is promising but challenging. Herein, Cd1-xCuxS materials have been synthesized and characterized by precise analytical tools. Cd1-xCuxS materials exhibited excellent photocatalytic activity for direct Red 23 (DR-23) dye degradation in visible light irradiation. The operational parameters, like dopant concentration, photocatalyst dose, pH, and initial concentration of dye were investigated during the process. The photocatalytic degradation process follows pseudo-first-order kinetics. As compared to other tested materials, 5% Cu doped CdS material revealed superior photocatalytic performance for the degradation of DR-23 (k = 13.96 × 10−3 min−1). Transient absorption spectroscopy, EIS, PL, and transient photocurrent indicated that adding copper to the CdS matrix improved the separation of photo-generated charge carriers by lowering the recombination rate. Spin-trapping experiments recognized the photodegradation primarily based on secondary redox products, i.e., hydroxyl and superoxide radicals. According to by Mott-Schottky curves, photocatalytic mechanism and photo-generated charge carrier density were elucidated regarding dopant-induced valence and conduction bands shifting. Thermodynamic probability of radical formation in line with the altered redox potentials by Cu doping has been discussed in the mechanism. The identification of intermediates by mass spectrometry study also showed a plausible breakdown mechanism for DR-23. Moreover, samples treated with nanophotocatalyst displayed excellent results when tested for water quality metrics such as DO, TDS, BOD, and COD. Developed nanophotocatalyst shows high recyclability with superior heterogeneous nature. 5% Cu-doped CdS also exhibit strong photocatalytic activity for the degradation of colourless pollutant bisphenol A (BPA) under visible light (k = 8.45 × 10−3 min−1). The results of this study offer exciting opportunities to alter semiconductors' electronic band structures for visible-light-induced photocatalytic activity for wastewater treatment.

Strong light-matter interaction and antireflection functionality of f-TiO2/GaN heterostructure broadband photodetector

Recent research focusing on broadband photodetectors offers a diverse range of applications like civilian, military, medical, etc. However, the available broadband photodetectors unable to retain high state-of-art performance parameters when operating at wide bandwidth. Herein, we report a broadband photodetector based on fibrils like TiO2 (f−TiO2) and GaN heterostructure. The TiO2 thin film has a high density of photo-generated charge carriers due to its great light absorption, antireflective, and highly oxidizing properties. These features of TiO2 enable a considerable increase in photocurrent for f−TiO2/GaN heterostructures compared to bare GaN, which provided a massive gain in the figure-of-merit performance metrics of f−TiO2/GaN-based devices. The fabricated f−TiO2/GaN device exhibits excellent optoelectronic characteristics, such as high responsivity (3.4 × 104 AW−1), outstanding external quantum efficiency (1.2 × 107 %), and detectivity (2.4 × 1012 Jones) under a weak optical signal of 90 nW. The design heterostructure-based photodetector also noticeably responds to illumination between the wavelengths of 250 nm and 780 nm. Additionally, TiO2 exhibits antimicrobial surface behavior, making it the best choice for pervasive biological and medical applications. Thus, TiO2/GaN heterostructure-based device like environmental screening, agriculture, medicine, and biodefense have proven their immense versatility in next-generation sequencing technology, medical science, cutting-edge crops, and creating interactive environments.

Correlating Charge Transfer Dynamics with Interfacial Trap States in High-Efficiency Organic Solar Cells.

The charge transfer between the donor and acceptor determines the photogenerated carrier density in organic solar cells. However, a fundamental understanding regarding the charge transfer at donor/acceptor interfaces with high-density traps has not been fully addressed. Herein, a general correlation between trap densities and charge transfer dynamics is established by adopting a series of high-efficiency organic photovoltaic blends. It is found that the electron transfer rates are reduced with increased trap densities, while the hole transfer rates are independent of trap states. The local charges captured by traps can induce potential barrier formation around recombination centers, leading to the suppression of electron transfer. For the hole transfer process, the thermal energy provides a sufficient driving force, which ensures an efficient transfer rate. As a result, a 17.18% efficiency is obtained for PM6:BTP-eC9-based devices with the lowest interfacial trap densities. This work highlights the importance of interfacial traps in charge transfer processes and proposes an underlying insight into the charge transfer mechanism at nonideal interfaces in organic heterostructures.

Photocarrier dynamics in Cd<sub>0.96</sub>Zn<sub>0.04</sub>Te measured by optical-pump terahertz-probe spectroscopy

Photogenerated carrier relaxation process and terahertz conductivity of Cd<sub>0.96</sub>Zn<sub>0.04</sub>Te are investigated by optical pump-terahertz probe spectroscopy at room temperature. With photoexcitation at 800 nm, the photogenerated carrier recovery process can be fitted with a single exponential curve, and its recovery time lasts several nanoseconds, which decreases with the increase of photogenerated carrier densities in a certain range of photogenerated carrier densities, relating to the radiative recombination of electron-hole pairs. The transient transmittance change of terahertz pulse remains the same with the photogenerated carrier densities increasing from 4.51×10<sup>16</sup> cm<sup>–3</sup> to 1.81×10<sup>17</sup> cm<sup>–3</sup>, which is because the number of loss carriers by defect trapping is approximate to the augment of carriers by photoexcitation. As the photogenerated carrier density increases from 1.81×10<sup>17</sup> cm<sup>–3</sup> to 1.44×10<sup>18</sup> cm<sup>–3</sup>, the magnitude of photoinduced absorption increases linearly with the increase of photogenerated carrier density due to the fact that most of the defects are occupied. When the photogenerated carrier densities are higher than 1.44×10<sup>18</sup> cm<sup>–3</sup>, the magnitude of photoinduced absorption remains almost the same, because the absorption of 800 nm pump pulse reaches a saturation level. The evolution of complex conductivity with photogenerated carrier density in a delay time of about 50 ps can be well fitted with Drude-Smith model. Our analysis provides an important data support and theoretical basis for designing and fabricating of Cd<sub>1–<i>x</i></sub>Zn<sub><i>x</i></sub>Te detection.

Excitation Intensity- and Size-Dependent Halide Photosegregation in CsPb(I0.5Br0.5)3 Perovskite Nanocrystals.

Although broad consensus exists that photoirradiation of mixed-halide lead perovskites leads to anion segregation, no model today fully rationalizes all aspects of this near ubiquitous phenomenon. Here, we quantitatively compare experimental, CsPb(I0.5Br0.5)3 nanocrystal (NC) terminal anion photosegregation stoichiometries and excitation intensity thresholds to a band gap-based, thermodynamic model of mixed-halide perovskite photosegregation. Mixed-halide NCs offer strict tests of theory given physical sizes, which dictate local photogenerated carrier densities. We observe that mixed-anion perovskite NCs exhibit significant robustness to photosegregation, with photosegregation propensity decreasing with decreasing NC size. Observed size- and excitation intensity-dependent photosegregation data agree with model predicted size- and excitation intensity-dependent terminal halide stoichiometries. Established correspondence between experiment and theory, in turn, suggests that mixed-halide perovskite photostabilities can be predicted a priori using local gradients of (empirical) Vegard's law expressions of composition-dependent band gaps.

Optoelectronic properties and ultrafast carrier dynamics of copper iodide thin films

As a promising high mobility p-type wide bandgap semiconductor, copper iodide has received increasing attention in recent years. However, the defect physics/evolution are still controversial, and particularly the ultrafast carrier and exciton dynamics in copper iodide has rarely been investigated. Here, we study these fundamental properties for copper iodide thin films by a synergistic approach employing a combination of analytical techniques. Steady-state photoluminescence spectra reveal that the emission at ~420 nm arises from the recombination of electrons with neutral copper vacancies. The photogenerated carrier density dependent ultrafast physical processes are elucidated with using the femtosecond transient absorption spectroscopy. Both the effects of hot-phonon bottleneck and the Auger heating significantly slow down the cooling rate of hot-carriers in the case of high excitation density. The effect of defects on the carrier recombination and the two-photon induced ultrafast carrier dynamics are also investigated. These findings are crucial to the optoelectronic applications of copper iodide.

Controllable interface engineering of g-C3N4/CuS nanocomposite photocatalysts

The rational fabrication of an efficient photocatalyst with optimal interface engineering remains a huge challenge for the enhancement of photocatalytic perpformance. Herein, a sequence of multidimensional nanocomposites with different spatial interfaces, including point, and line or face-contact surface are controllable designed by horizontal growing diverse dimensional (0D, 1D,2D and 3D) copper sulfide (CuS) on 2D graphitic-carbon nitride (g-C 3 N 4 ) nanosheets. Physical and photochemical measurements demonstrate that the formation of an extraordinary 2D/2D face-to-face contact interface can not only enhance the specific surface area, visible light utilization, and photo-excited charge separation efficiency, but also elevate the density and lifetime of photo-generated carriers. The results of ultraviolet photoemission spectroscopy (UPS) and density functional theory (DFT) calculation also confirm that charge transfer tends to occur in face-to-face contact. Among the types of nanocomposites considered, 2D/2D g-C 3 N 4 /CuS has the smallest electron transfer barrier (Ф Be ) between active species, thereby displaying the maximum photocatalytic apparent rate constant, which is about 12 times larger than that of pristine g-C 3 N 4 . Most importantly, this work systematically investigates the relationship between microscopic interface structure and photocatalytic activity from the perspective of the optical, and electrical and energy levels, providing a new insight on the rational design of desired interface engineering towards efficient photocatalysts. • 2D/xD g-C3N4/CuS nanocomposites with different spatial interfaces were fabricated. • 2D/2D g-C3N4/CuS nanocomposite displays the optimal photocatalytic performance. • The concentration, lifetime and separation-efficiency of photogenerated carriers were tested. • The energy barrier between active species and catalysts was analyzed by UPS and DFT. • The photocatalytic mechanism of 2D/2D g-C3N4/CuS nanocomposites is proposed.

Enhancing performance of tin-based perovskite solar cells by polyvinyl pyrrolidone doping strategy

The power conversion efficiency (PCE) of tin-based perovskite solar cells (TPSCs) was significantly improved when polyvinyl pyrrolidone (PVP) was doped into the formamidinium tin iodide (FASnI3) layer with an optimized doping mass. After doping, the FASnI3 grain size, film smoothness and average carrier lifetime were largely enhanced, which demonstrate the crystallization quality improvement to FASnI3. The champion PCE of the enhanced TPSC was improved to 7.83%, which is relatively 27% larger than that of the control, 6.15%. After continuous storage for 15 days, the PCE of the enhanced TPSC remained 74.4% of its original value. Using the finite difference time domain (FDTD) simulation method, we demonstrated that the light absorption ratio of the FASnI3:PVP layer was larger than that of the FASnI3 , and the photo-generated carrier density of the enhanced TPSC was approximately 2% ∼ 3% larger than that of the control. The ultimate efficiency of the enhanced TPSC was calculated to be 26.23%. It is potential to dope PVP into FASnI3 to further enhance the device performance of TPSCs.

Piezo-assisted photoelectric catalysis degradation for dyes and antibiotics by Ag dots-modified NaNbO3 powders

NaNbO3 is explored as a potential candidate for catalysis due to its excellent piezoelectricity. However, its photocatalysis is significantly limited for the inherent characteristics of wide band gap. In this work, NaNbO3 and Ag dots-modified NaNbO3 micron-sized powders are prepared and applied in piezo-photocatalysis to explore the coupling effects. The results show that the piezocatalysis degradation efficiency of RhB solution (the initial concentration C0 = 5 mg/L) reaches 92.3% within 30 min, and the rate constant k is 0.0889 min−1. The efficiency of piezo-photocatalysis degradation reaches 92.7% in 25 min, and the rate constant k is 0.11 min−1 for NaNbO3. The rate constant k of Ag dots-modified NaNbO3 for piezo-photocatalytic degradation of RhB (C0 = 5 mg/L) is increased to 0.16314 min−1. The optical and electrochemical activities of NaNbO3-0.75hAg are studied to reveal the role of Ag nanoparticles. The band gap of NaNbO3 is 3.59 eV, which is decreased to 3.36 eV after the modification with Ag dots. The lower band gap means that e− in the valence band is more easily excited to shift the conduction band, which is beneficial to the photocatalytic reaction process. In addition, NaNbO3-0.75hAg has a higher photogenerated carrier density, a faster electron-hole pair separation and transfer rate, which are beneficial to the catalytic oxidation process. The work provides a useful means to decease the band gap of NaNbO3 and achieve outstanding piezo-photocatalysis for degradation of hazardous organic dyes in contaminated water.

Circular photogalvanic effect of surface states in the topological insulator Bi2(Te0.23Se0.77)3 nanowires grown by chemical vapor deposition

Circular photogalvanic effect (CPGE) of single-crystalline ternary topological insulator Bi2(Te0.23Se0.77)3 nanowires, which are synthesized by the chemical vapor deposition, have been investigated. It is demonstrated that the distributions of the elements in the nanowires are fairly uniform, and they have high crystal quality. Compared with Bi2Se3 nanowires, the ternary Bi2(Te0.23Se0.77)3 nanowires have better responsivity to circularly polarized light. The incident angle dependence of the CPGE current indicates that the symmetry of the surface states of the nanowire belongs to C3v symmetry. The temperature dependence of the CPGE current is also investigated. As the temperature decreases from 300 to 77 K, the CPGE current first increases and then decreases, which is due to the variation of the mobility and photo-generated carrier density with temperature. Our work suggests that ternary Bi2(Te1−xSex)3 nanowires are good candidates for designing polarization-sensitive photoelectric devices.

Photogenerated carrier density dependence of ultrafast carrier dynamics in intrinsic 6H-SiC measured by optical-pump terahertz-probe spectroscopy

Ultrafast carrier dynamics and terahertz conductivity of intrinsic 6H-SiC are investigated by optical-pump terahertz-probe spectroscopy at room temperature. The amplitude of photoinduced absorption increases gradually and followed by a biexponential recovery with photogenerated carrier densities from 7.43 × 1017 to 2.81 × 1018 cm −3 at 400 nm. With the increase of photogenerated carrier density, the fast decay time decreases may be due to higher order recombination (i.e., Auger recombination), and the slow one increases is related to the band-filling effect. The evolution of complex conductivity with photogenerated carrier density at the delay time of about 50 ps can be better fitted with Drude–Smith​ model than Drude–Lorenz model. The Drude–Smith scattering time τDS and Smith​ parameter c1 increase with photogenerated carrier density implies carriers backscattering is dominant. Our analysis provides important basic data for research and improvement ultrafast semiconductor devices of intrinsic 6H-SiC.

Insight into a novel microwave-assisted W doped BiVO4 self-assembled sphere with rich oxygen vacancies oriented on rGO (W-BiVO4-x/rGO) photocatalyst for efficient contaminants removal

A novel W doped BiVO4 with rich oxygen vacancies (OVs) oriented on reduced graphene oxide (rGO) photocatalyst (W-BiVO4-x/rGO) was developed by a simple one-step microwave hydrothermal method for efficient removal of ciprofloxacin (CIP) and chlortetracycline (CTC) under visible light. The synergistic effect of W doping, BiVO4 with OVs, and rGO on the enhancement of photocatalytic activity was clarified from experimental and Density Functional Theory (DFT) calculations results. W doping significantly improved the density of photo-generated charge carriers. The introduction of rGO enlarged the specific surface area, expanded the light absorption range, and promoted the charge transfer of BiVO4. OVs both narrowed the band gap and enhanced the generation of O2−. W doping and OVs both enhanced the interactions between BiVO4 and rGO. The CIP degradation rate constant of W-BiVO4-x/rGO photocatalyst was 1.73, 5.32, 6.43, and 10.70 times higher than those of W-BiVO4/rGO, BiVO4/rGO, W-BiVO4, and BiVO4, respectively. Besides, W-BiVO4-x/rGO photocatalyst prepared by microwave hydrothermal method exhibited higher photocatalytic activity and better combination between materials. This study offers a new strategy to address the problem of antibiotic residues in wastewater.

Atomlike interaction and optically tunable giant band-gap renormalization in large-area atomically thin MoS2

Coulomb interactions in atomically thin transition metal dichalcogenides can be dynamically engineered by exploiting the dielectric environment to control the optical and electronic properties. Here we demonstrate an optically tunable giant band-gap renormalization (BGR) $\ensuremath{\sim}1200$ and 850 meV from the edge of the conduction band and complete suppression of the exciton absorption in large-area single-layer (1L) and three-layer (3L) ${\mathrm{MoS}}_{2}$, respectively. The observed giant BGR is two orders of magnitude larger than that in the conventional semiconductors, and it persists for tens of ps. Strikingly, our results demonstrate photoinduced transparency at the electronic band gap using an intense optical field at room temperature. Exciton bleach recovery in 1L and 3L show a contrasting fluence-dependent response, demonstrating the layer-dependent optical tuning of exciton lifetime in a way that would be both reversible and real time. We find that the optical band gap (exciton resonance peak) shows a transient redshift followed by an anomalous blueshift from the lowest energy point as a function of the photo-generated carrier density. The observed exciton energy shift is analogous to atom-atom interactions, and it varies as a Lennard-Jones like potential as a function of the interexciton separation.