Free Exciton Transitions Research Articles

Deep‐Ultraviolet Luminescence Properties of AlN

High‐resolution, low‐excitation photoluminescence (PL) spectroscopy is performed for unintentionally doped, silicon‐doped, and magnesium‐doped homoepitaxial aluminum nitride (AlN) films, using a wavelength‐tunable high‐repetition‐rate laser. The wavelength‐tunable laser is used to distinguish between the luminescence and scattering signals from AlN. Providing the high‐resolution, low‐excitation PL spectra, the current understanding of the deep‐ultraviolet luminescence properties of AlN is reviewed and potential assignments for the unknown luminescence lines and bands are discussed. Although previous studies have led to a consensus on the origins of some emission peaks and bands such as the neutral silicon donor‐bound exciton transition and free exciton transitions involving longitudinal optical phonons, it is shown that many of the emission peaks are still unidentified. The origins of all the emission peaks should be elucidated to enable control of the electronic and optoelectronic properties of AlN.

Recombination dynamics and pronounced re-absorption effect in ZnO single crystals under two-photon excitation

Time-resolved photoluminescence measurements were conducted on ZnO single crystals using two-photon (2P) excitation at low temperatures ranging from 25 to 260 K. The decay constants of various emission peaks are determined and compared between one-photon and two-photon excitations using a recently modified localized-state ensemble model. The analysis is focused on the redshift of the phonon-assisted free exciton transition energy with increasing temperature. A significantly longer radiative recombination lifetime of approximately 840 ps is observed under 2P excitation. The extended lifetime of excitons during 2P excitation is achieved through the interplay of multiple mechanisms. Photon-recycling, enabled by high absorption coefficients and total internal reflection, facilitates re-absorption and new carrier generation. Phonon scattering lowers photon energy, allowing their escape from the crystal, while exciton–polariton interactions delay photon travel, collectively contributing to the extension of exciton lifetimes. The re-absorption effect and elongated carrier lifetime observed from our work could be beneficial for applications of ZnO materials in 2P imaging, particularly of biological samples.

Intensive and broad bound exciton emission at cryogenic temperature in suspended monolayer transition metal dichalcogenides

Two-dimensional (2D) transition metal chalcogenides (TMDCs) are expected to play an important role in next-generation optoelectronic devices. However, the properties of 2D TMDCs can be largely affected by defects, and thus, many efforts have been made to characterize and eliminate the defects. Here, we investigate defect-related trap states, particularly in large-area suspended ${\mathrm{WS}}_{2}$ monolayers at cryogenic temperatures, using steady-state and time-resolved photoluminescence (TRPL) spectroscopy. We observed an intensive and broad (full width half maximum $\ensuremath{\sim}170\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$) photoluminescence (PL) emission centered at 1.83 eV, 0.3 eV lower than the free exciton transition energy. From temperature-dependent PL spectra, the thermal activation energy of excitons escaping from trap states is determined to be 30 meV. The TRPL spectrum shows an ultralong lifetime at 13 K. The long-lived emission with the broad PL linewidth is attributed to excitons bound to defects of possibly chalcogenide-site substitution and sulfur vacancies. Compared with the supported monolayer, the defect-related trap states are found to be deeper in the suspended layer and can trap excitons more efficiently due to less dielectric screening. In this letter, we provide further understanding of trap states and decay channels in monolayer TMDCs and show that suspended 2D TMDCs can provide a pure platform for studying defect-related properties.

The effect of magnesium doping concentration on the structural and optical properties of zinc oxide nanoparticles

In this study, the optical and structural properties of Magnesium doped Zinc Oxide (Zn1-xMgxO) nanoparticles (x = 1%, 3%, and 5%) were examined. The synthesis was carried out at 300 oC by using a simple co-precipitation method. The structural samples were used to characterize the synthesized nanoparticles by using Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD). Energy Dispersive X-Ray spectroscopy (EDX) was used to determine the chemical composition of the samples. Furthermore, UV-Vis spectroscopy was conducted to study our samples’ optical characteristics. As per XRD analysis, both pure and Mg doped ZnO samples possess wurtzite structure. The particle size was revealed to decrease significantly with Mg concentration, from 36 nm for pure sample to 23.87 nm with 5% Mg. In comparison to ZnO, the XRD pattern of Mg-doped ZnO shows a peak shift towards lower 2θ values. As a result of deformation of crystal structure caused by integrated Mg+2 ions onto Zn+2 sites. SEM images of synthesized samples reveal that the nanoparticles possess sheet, spherical and rod like morphologies for different values of x. The EDX analysis verifies the purity of samples within the detection limits. According to optical absorption spectra obtained from UV-Visible spectroscopy in the 200nm to 1000nm range affirmed that as Mg concentration increased, the band gap increased from 3.37 eV for pure sample to 3.74 eV with 5% Mg. The UV-Visible spectra of pure ZnO and Mg-doped ZnO samples also exhibit distinct peaks in the UV region at 335 nm, 311 nm, 310.6 nm, and 310 nm. Across the spectra of pure and Mg-doped ZnO samples, the UV peak is attributed to free exciton transitions, whereas, in the spectra of Mg-doped ZnO UV absorbance peak in the visible region is attributed to the radiative transitions of electrons captured at oxygen vacant sites with holes trapped at singly ionized oxygen vacancies.

Low Temperature Photoluminescence Properties of α‐CsPbI 3 Nanocrystals with High Quantum Yield

Temperature-dependent photoluminescence (PL) properties of high quality cubic phase α-CsPbI3 nanocrystals are investigated, and thermal transfer from free excitons to trapped excitons is observed. Free exciton recombination is the main origin of the high PL quantum yield (97%) at room temperature. Little change of PL peak energy is observed with decreasing temperature from 280 to 200 K, and it shifts from 1.84 to 1.81 eV in the region of 200 and 80 K. The PL peak intensity decreases with decreasing temperature first and then increases, and the minimum value is observed at 180 K. Two PL bands are obtained by Gaussian fittings in the PL spectra detected at different temperatures, and the high energy band is attributed to the emission of free excitons, while that for the low energy band is related to the transition of trapped excitons. The PL integrated intensity ratio of trapped exciton and free exciton transition increases from 0.18 to 2.10 with decreasing the temperature, which suggests that thermal transfer from free exciton to trapped exciton states leads to the dominant optical emission of trapped exciton recombination in the low temperature region.

Orientation-Mediated Luminescence Enhancement and Spin-Orbit Coupling in ZnO Single Crystals.

Temperature-, excitation wavelength-, and excitation power-dependent photoluminescence (PL) spectroscopy have been utilized to investigate the orientation-modulated near band edge emission (NBE) and deep level emission (DLE) of ZnO single crystals (SCs). The near-band-edge emission of ZnO SC with <0001> orientation exhibits strong and sharp emission intensity with suppressed deep level defects (mostly caused by oxygen vacancies Vo). Furthermore, Raman analysis reveals that <0001> orientation has dominant E2 (high) and E2 (low) modes, indicating that this direction has better crystallinity. At low temperature, the neutral donor-to-bound exciton (DoX) transition dominates, regardless of the orientation, according to the temperature-dependent PL spectra. Moreover, free-exciton (FX) transition emerges at higher temperatures in all orientations. The PL intensity dependence on the excitation power has been described in terms of power-law (I~Lα). Our results demonstrate that the α for <0001>, <1120>, and <1010> is (1.148), (1.180), and (1.184) respectively. In short, the comprehensive PL analysis suggests that DoX transitions are dominant in the NBE region, whereas oxygen vacancies (Vo) are the dominant deep levels in ZnO. In addition, the <0001> orientation contains fewer Vo-related defects with intense excitonic emission in the near band edge region than other counterparts, even at high temperature (~543 K). These results indicate that <0001> growth direction is favorable for fabricating ZnO-based highly efficient optoelectronic devices.

Stimulation and Enhancement of Near‐Band‐Edge Emission in Zinc Oxide by Distributed Bragg Reflectors

AbstractThe free exciton transition (near‐band‐edge emission, NBE) of ZnO at ≈388 nm can be strongly enhanced and even stimulated by an underlying photonic structure. 1D Photonic crystals, so‐called distributed Bragg reflectors, are utilized to suppress the deep‐level emission of ZnO (DLE, ≈500–530 nm). The reflector stacks are fabricated in a layer‐by‐layer procedure by wet‐chemical synthesis. They consist of low‐ε porous SiO2 layers and high‐ε TiO2 layers. Varying the thickness of the SiO2 layers allows tuning the optical bandgap in a wide range between ≈420 and 800 nm. A ZnO layer is deposited on top of the reflector stacks by sol–gel synthesis. The spontaneous photoluminescence (PL) emission of the ZnO film is modulated by the photonic structure. When the optical bandgap of the reflector is in resonance with the deep‐level emission of ZnO (DLE, ≈500–530 nm), then this defect‐related emission mode is suppressed. Strong NBE emission is observed even when the ZnO layer does not show any NBE emission (due to low crystallinity) in the absence of the photonic structure. With this cost‐efficient synthesis method, emitters for, e.g., luminescent gas sensors can be fabricated.

Low-Temperature Chemical Vapor Deposition Growth of MoS2 Nanodots and Their Raman and Photoluminescence Profiles

We report on the growth of an ordered array of MoS2 nanodots (lateral sizes in the range of ∼100–250 nm) by a thermal chemical vapor deposition (CVD) method directly onto SiO2 substrates at a relatively low substrate temperature (510–560°C). The temperature-dependent growth and evolution of MoS2 nanodots and the local environment of sulfur-induced structural defects and impurities were systematically investigated by field emission scanning electron microscopy, micro-Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) techniques. At the substrate temperature of 560°C, we observed mostly few-layer MoS2, and at 510°C, multilayer MoS2 growth, as confirmed from the Raman line shape analysis. With reduced substrate temperature, the density of MoS2 nanodots decreases, and layer thickness increases. Raman studies show characteristic Raman modes of the crystalline MoS2 layer, along with two new Raman modes centered at ∼346 and ∼361 cm−1, which are associated with MoO2 and MoO3 phases, respectively. Room temperature photoluminescence (PL) studies revealed strong visible PL from MoS2 layers, which is strongly blue-shifted from the bulk MoS2 flakes. The strong visible emission centered at ∼ 658 nm signifies a free excitonic transition in the direct gap of single-layer MoS2. Position-dependent PL profiles show excellent uniformity of the MoS2 layers for samples grown at 540 and 560°C. These results are significant for the low-temperature CVD growth of a few-layer MoS2 dots with direct bandgap photoluminescence on a flexible substrate.

Size-induced evolution of optical properties in gallium selenide thin layers

Thin layers of gallium selenide (GaSe) have been fabricated on Si:SiO2 substrates by micromechanical exfoliation. Size-induced effects in GaSe thin layers of various thickness have been revealed. The photoluminescence maximum associated with direct free exciton transition shifts to higher energies with reduction of GaSe crystal thickness. When going from the bulk GaSe crystal down to 6 tetralayers, the bandgap value increases from 2 eV (620 nm) to 2.17 eV (571 nm). No photoluminescence signal has been detected for GaSe layers thinner than 6 TL. At the same time, the Raman spectra of thin GaSe flakes demonstrate the features corresponding to Ga2Se3, Ga2O3 and other compounds. The appearance of these lines is attributed to thermal and photo-induced oxidation of GaSe layers. For the thin flakes, a remarkable modification of the photoluminescence spectra shape takes place. Presumably, it is a consequence of the electronic structure evolution, as the top of the valence band has undergone changes and has flattened for 8 TL.

Recovering the optical transitions in tin oxide thin films at room temperature using electroreflectance

Observing the optical transitions in Tin Oxide at room temperature is a difficult task because of its forbidden bandgap nature in bulk form. Moreover, it possesses the well know low emission efficiency issue in its reduced dimensional thin film structures . In this work, we use Electroreflectance (ER) method to recover the optical transitions in tin oxide thin films . A metal-oxide-metal geometry, with SnO2 film was sandwiched between two metal electrodes , was illuminated by the light under an external perturbation voltage. This results the well-enhanced critical points in the ER spectra with a suppressed featureless background. The characteristics of observed critical points are found using the third derivative functional form model. The points are identified as defect transition and surface exciton transition originated due to the presence of defect states formed by the oxygen vacancies . In addition to this, the free exciton transition, from deep of the valence band to the conduction band , is explicitly revealed by ER spectra of r-SnO 2 film. A built-in electric field driven by external voltage is supposed to be the main factor for enhancing the critical points associated with optical transitions. • The optical transitions in SnO 2 films were observed at room temperature by using electroreflectance method. • A metal-oxide-metal structure was fabricated to explore the optical transitions in SnO 2. • The electroreflectance method with external perturbation of 10 V was applied across the geometry. • The electroreflectance spectra revealed the defect and exciton transitions within the visible, near UV, and UV region.

Combination of optical transitions of polarons with Rashba effect in methylammonium lead trihalide perovskites under high magnetic fields

We investigate photoluminescence (PL) transitions of ${\text{MA}\text{Pb}X}_{3}$ ($X=\text{I}$, Br, and Cl) organic-inorganic hybrid perovskite single crystals under magnetic fields of up to 60 T. In these materials, sharp free-exciton transition peaks emerge at a low temperature (4.2 K). Under strong magnetic fields, the free-exciton PL transitions of three different halogens show dramatic differences. The free-exciton transitions of the ${\mathrm{MAPbCl}}_{3}$ crystal undergo negative energy shifts, while those of the ${\mathrm{MAPbBr}}_{3}$ crystal show normal diamagnetic shifts. To obtain the variation from Cl to Br, we attempt to measure PL transitions of ${\mathrm{MAPbCl}}_{x}{\mathrm{Br}}_{3\ensuremath{-}x}$. For ${\mathrm{MAPbI}}_{3}$, the transition-energy shifts for both ${\ensuremath{\sigma}}^{+}$ and ${\ensuremath{\sigma}}^{\ensuremath{-}}$ transitions at 4.2 K exhibit a power-law dependence on the magnetic field. Such inconsistent magnetic-field effects on different halogens make it difficult to understand the transition-energy behavior through a unified model. We propose a possible mechanism for the field effects that is based on a combination of the Rashba effect induced by strong spin-orbit coupling and the polaron effect caused by the polar nature of the inorganic elements.

Correlative Study of Enhanced Excitonic Emission in ZnO Coated with Al Nanoparticles using Electron and Laser Excitation

Recently, metal nanoparticle surface coatings have been found to significantly enhance the ultra-violet luminescence intensity from ZnO, providing a viable means to mitigate optical losses and improve LED performance. Although there is general agreement that resonantly excited Localized Surface Plasmons (LSPs) in metal nanoparticles can directly couple to excitons in the semiconductor increasing their spontaneous emission rate, the exact mechanisms involved in this phenomenon are currently not fully understood. In this work, LSP-exciton coupling in bulk and nanostructured ZnO coated with a 2 nm Al nanoparticle layer is investigated using correlative photoluminescence and depth-resolved cathodoluminescence and time-resolved photoluminescence spectroscopy. Temperature-resolved cathodoluminescence and photoluminescence measurements from 10 K to 250 K show free exciton (FX) emission enhancement factors up to 12x at 80 K, and reveal that the FX couple more efficiently to the LSPs compared to the localized donor-bound excitons. A strong polarization dependence between the LSPs and FX is observed where FX transitions are more strongly enhanced when polarized in the same direction as the electric field of the incident excitation, which is different for laser and electron beam sources. This result indicates that selective enhancement of the excitonic emission peaks in the ZnO coated with Al nanoparticles can be achieved by choosing the appropriate ZnO substrate orientation.

Observation of bandgap renormalization in mesoscopic zinc oxide particles

Deep UV photoluminescence (PL) spectroscopy was employed to study the optical properties of mesoscopic zinc oxide (ZnO) particles using the third harmonic laser (λ ~ 260 nm) generated from a Ti: sapphire pulse laser for optical excitation. PL spectrum measured at 15 K exhibits excitonic emissions at 3.362 and 3.375 eV due to the donor bound exciton (DoX) and free exciton (FX) transitions, respectively. The dominant emission peak is observed at around 3.308 eV due to the recombination of free electrons from the conduction band to the holes bound to an acceptor state (FA) along with longitudinal optical (LO) phonon replicas. From the power dependent PL measurement, we observed a redshift of FX and DoX emission peaks with increasing excitation laser power, showing a bandgap renormalization effect. Temperature dependent PL measurements were also performed to explore the bandgap renormalization effect. Fitting of our power dependent PL data with Vashishta, and Kalia model shows the clear trend in a good agreement with the model.

Effects of NH3 flow on structural and optical properties of ZnO films grown by atmospheric-pressure chemical vapor deposition

Structural and optical properties of zinc oxide (ZnO) films doped with nitrogen (N) grown on r-plane sapphire substrates by the atmospheric-pressure chemical vapor deposition using zinc, water, and ammonia (NH3) as source materials have been studied. The small amount of NH3 flow was found to be effective for improving the film orientation and surface morphology. Photoluminescence (PL) spectra of the as-grown undoped and N-doped ZnO films were composed of a near-band-edge emission with two peaks at ~376 and ~386 nm and a green band (GB) emission with a peak at ~540 nm. PL and PL excitation (PLE) measurements for the as-grown N-doped films grown at the different NH3 flow rates indicated the possibility of the contribution of the N and/or hydrogen (H) related defect to both the appearance of the peak at ~386 nm and the excitation process of the GB emission. The comparison between the PL and photoacoustic (PA) spectra showed that the N atom occupying oxygen site (NO) or its complex defect is the most conceivable candidate for the non-radiative center responsible for the intra-band-gap PA absorption. After post annealing under the different conditions, a red band (RB) emission appeared at ~720 nm besides the GB emission. Both the RB and GB emissions observed for the annealed films were found to be effectively excited by the band-to-band (inter-band-gap) and free exciton transitions. Regardless of the difference in annealing condition, all the PLE spectra for the RB emissions showed a weak excitation band extending from 460 nm to 680 nm, suggesting the formation of the deep impurity level probably due to the NO acceptor. It was also confirmed by X-ray photoelectron spectroscopy that the NO deep acceptors were formed in the films, but passivated by the H atoms and/or compensated by the H related donors and/or by the nitrogen molecule substituting oxygen site [(N2)O] double donors.

Insights into the photoluminescence properties of gel-like carbon quantum dots embedded in poly(methyl methacrylate) polymer

Carbon dots (CDs) have become one of the most investigated nanomaterials in the last few years, since they offer excellent prospects to be used in a wide range of technological applications. Nonetheless, the understanding of the photoluminescence (PL) processes remains unclear and solely based on spectroscopic measurements at room temperature, which hampered a full discussion on the nature of recombination mechanisms. Insights into the CDs’ PL and corresponding recombination models can be achieved from temperature, excitation energy and excitation density dependency studies. Thus, in this paper, CDs with a mean diameter of ∼3.0 nm were embedded in poly(methyl methacrylate) (PMMA) and studied by such optical techniques. The recombination of CDs/PMMA composites was found to be due to an overlap of emitting centres, both in the blue and green regions, with intensity and peak position strongly dependent on the excitation density. Moreover, this emission was found to be preferentially populated by two overlapped excitation bands. Temperature-dependent PL measurements corroborate the hypothesis of overlapped emitting optical centres (e.g. small molecules at the CDs’ surface), as no shift on peak position and no changes in spectral shape were observed, which allow to exclude the hypothesis of a conventional free excitonic transition and quantum confinement effects, as observed in typical nanosized semiconductors with dimensions lower than the exciton Bohr radius.

Time-resolved photoluminescence and photoreflectance spectroscopy of GaN layers grown on SiN-treated sapphire substrate: Optical properties evolution at different growth stages

In this paper, we present a systematic study of the optical properties evolution of GaN films during the complete growth process on SiN-treated sapphire substrates by atmospheric pressure metalorganic vapor phase epitaxy. The growth process was monitored using in-situ laser reflectometry and was interrupted at different stages to obtain the studied samples. The obtained samples were ex-situ characterized by means of photoluminescence (PL), photoreflectance (PR) and time-resolved PL (TRPL) spectroscopies. The PL emission from the samples of the initial growth stages originates from nano-crystallite and defect states due to the 3D growth mode. However, with increasing layer thickness, the 2D growth mode is established, and the PL spectrum is dominated by free-exciton emission. The electric field extracted by applying the Franz-Keldysh oscillation (FKO) theory on the PR spectra shows a trend to decrease as the GaN layer thickness is increased. For fully coalesced layers, the FKO totally disappears, and the PR spectrum is dominated by free-exciton transitions. TRPL measurements demonstrate the contribution of two processes to the PL decay, i.e., fast and slow components. While the slow decay time reveals the same sensitivity to different types of dislocations (twist and tilt mosaics), the fast decay time is more affected by the twist mosaic than by the tilt one.

Effects of thermal ionized-impurities and mosaicity on the excitonic properties of GaN grown by MOVPE

In this work, the effects of thermal ionized-impurities and mosaicity on the excitonic properties of GaN are systematically investigated. For this reason, a number of GaN epilayers with different impurity/defect concentrations have been grown on c-plane sapphire substrates by atmospheric pressure metalorganic vapor phase epitaxy (MOVPE). Variable-temperature photoreflectance (PR) spectroscopy is used, in the temperature range from 10K to 300K, to study the thermal broadening of excitonic transitions. A comprehensive comparison between the experimental data and the available theoretical models is presented. The results show that the theoretical models, in which the exciton scattering with thermal-ionized impurity is ignored, fail to account for the experimental data. However, when the exciton-ionized impurity interaction is considered, the simulation results become in good agreement with the experimental data. Based on the last approach, the parameters that describe the thermal broadening of the exciton linewidth are accurately evaluated and discussed. It is found that the exciton-LO phonon coupling strength, which suffers from scattering in the literature, is almost constant for the different samples (ΓLO≈255±15meV), and it is independent to impurity concentration. Furthermore, it is observed that the free exciton transitions have not the same sensitivity to the tilt and twist mosaic interaction, and the C-exciton undergoes a stronger dislocation scattering than FXB and FXA.

New Evidence for the Second Conduction Band in 4H SiC

Differential absorption measurements were taken on ultra-pure boule pieces and epitaxial films of 4H SiC. The energy range of particular interest is from 3.40 eV to 3.52 eV. The free exciton energy gap associated with the second lowest conduction band at the M point in the Brillouin zone was determined to be EGX-2 = 3.4107 eV. This value is obtained from phonon assisted free exciton transitions involving the second conduction band measured in transmission with polarization E⊥c. The energy separation of the two lowest conduction bands is determined to be 144 ± 2 meV. Some replica peaks attributable to the spin orbit splitting in the valence band are also seen.

Probing the exciton-phonon coupling strengths of O-polar and Zn-polar ZnO wafer using hard X-ray excited optical luminescence

The temperature-dependent hard X-ray excited optical luminescence (XEOL) spectroscopy was used to study the optical properties of O and Zn polarity of a c-plane single crystal ZnO wafer. By analyzing the XEOL and XRD, we found an unprecedented blue shift of the free exciton transition with increasing the excited carrier density as tuning the X-ray energy across the Zn K-edge, and the O-polar face possesses better crystal structure than the Zn-polar one. This spectral blue shift is attributed to the Coulomb screening of the spontaneous polarization by the excited free carriers that result in decreasing the exciton-phonon Fröhlich interaction to reduce exciton binding energy.

(Invited) Photocurrent Spectroscopy of Exciton and Free Particle Optical Transitions in Suspended Carbon Nanotube Pn-Junctions

We study photocurrent generation in individual, suspended carbon nanotube pn-junction diodes formed by electrostatic doping using two gate electrodes. Photocurrent spectra collected under various electrostatic doping concentrations reveal distinctive behaviors for free particle optical transitions and excitonic transitions. In semiconducting nanotubes, the photocurrent generated by excitonic transitions exhibits a strong gate doping dependence, while that of the free particle transitions is gate independent.1 Here, the built-in potential of the pn-junction is required to separate the strongly bound electron-hole pairs of the excitons, while free particle excitations do not require this field assisted charge separation. We observe a sharp, well defined E11 free particle interband transition in contrast with previous photocurrent studies. Several steps are taken to ensure that the active charge separating region of these pn-junctions is suspended off the substrate in a suspended region that is substantially longer than the exciton diffusion length and, therefore, the photocurrent does not originate from a Schottky junction. We present a detailed model of the built-in fields in these pn-junctions, which, together with phonon-assistant exciton dissociation, predicts photocurrents on the same order of those observed experimentally. In addition, we investigate the competing photocurrent mechanisms in quasi-metallic nanotubes over a wide range of bandgaps (30–180 meV).2 Here, we correlate homogenous pn-doping dependence ( Vg1 = – Vg2 ) of the photocurrent with the current–voltage ( I–Vg ) characteristics (Vg1 = Vg2 ) of each device. These I–Vg characteristics, in turn, are used to determine the bandgap of the nanotubes by fitting our data to a Landauer transport model. The mechanism underlying photocurrent generation is then established by correlating the photocurrent transport with the calculated thermoelectric power, the measured conductance, and the extracted bandgap for all devices. 1. Chang, S.W., J. Theiss, J. Hazra, M. Aykol, R. Kapadia and S.B. Cronin, Photocurrent spectroscopy of exciton and free particle optical transitions in suspended carbon nanotube pn-junctions. Applied Physics Letters, 107, 053107 (2015). 2. Amer, M.R., S.W. Chang and S.B. Cronin, Competing Photocurrent Mechanisms in Quasi-Metallic Carbon Nanotube pn Devices. Small, 11, 3119-3123 (2015).