Rongtao Lu

Suspending single-wall carbon nanotube thin film infrared bolometers on microchannels

Suspended single-wall carbon nanotube (SWCNT) thin film bolometers have been fabricated on microchannels patterned on Si substrates using electron-beam lithography. The much improved bolometric photoresponse is attributed to the reduced thermal link between SWCNT bolometer and substrate, which can be controlled by tuning the width and spacing of the microchannels. The detectivity D∗ up to 4.5×105 cm Hz1/2/W has been obtained at room temperature, which is at least five times better than that of the unsuspended counterpart and may be further improved via elimination of metallic SWCNTs and improvement of the charge and heat transport across the intertube junctions.

Extraordinary photocurrent harvesting at type-II heterojunction interfaces: toward high detectivity carbon nanotube infrared detectors.

Despite the potentials and the efforts put in the development of uncooled carbon nanotube infrared detectors during the past two decades, their figure-of-merit detectivity remains orders of magnitude lower than that of conventional semiconductor counterparts due to the lack of efficient exciton dissociation schemes. In this paper, we report an extraordinary photocurrent harvesting configuration at a semiconducting single-walled carbon nanotube (s-SWCNT)/polymer type-II heterojunction interface, which provides highly efficient exciton dissociation through the intrinsic energy offset by designing the s-SWCNT/polymer interface band alignment. This results in significantly enhanced near-infrared detectivity of 2.3 × 10(8) cm·Hz(1/2)/W, comparable to that of the many conventional uncooled infrared detectors. With further optimization, the s-SWCNT/polymer nanohybrid uncooled infrared detectors could be highly competitive for practical applications.

Development of Nanopatterned Fluorine-Doped Tin Oxide Electrodes for Dye-Sensitized Solar Cells with Improved Light Trapping

Transparent conductors (TCs) are an important component of optoelectronic devices and nanoscale engineering of TCs is important for optimization of the device performance through improved light trapping. In this work, patterned periodic arrays of nanopillars and nanolines of pitch size of ~700 nm were created on fluorine-doped tin oxide (FTO) using nanoimprint lithography and reactive ion etching using environmentally friendly gases. The patterned FTO exhibits enhanced light trapping as compared to the unpatterned FTO, which agrees well with simulations based on Finite-Difference Time-Domain method for up to a distance of 4 μm. Dye sensitized solar cells (DSSCs) fabricated on the patterned FTO exhibited improved performance (fill factor and power conversion efficiency), which can be attributed to enhanced light absorption in the range 400-650 nm. Further, electrochemical impedance measurements revealed lower recombination resistance for the patterned FTO/TiO(2) electrode compared to the unpatterned FTO electrode/TiO(2) electrode as a result of better light capturing properties of patterned FTO. The direct fabrication of nanopatterns on TCs developed in the present study is expected to be a viable scheme for achieving improved performance in many other optoelectronic devices.

High performance multiwall carbon nanotube bolometers

High infrared bolometric photoresponse has been observed in multiwall carbon nanotube (MWCNT) films at room temperature. The observed detectivity D∗ in exceeding 3.3×106 cm Hz1/2/W on MWCNT film bolometers is a factor of 7 higher than that obtained on the single-wall CNT (SWCNT) counterparts. The response time of about 1–2 ms on MWCNT bolometers is more than an order of magnitude shorter than that of SWCNT bolometers. The observed high performance may be attributed to the naturally suspended inner-shell structure in a MWCNT, which enhances photon absorption and restricts bolometer external thermal link to environment.

Development of pulsed laser deposition for CdS/CdTe thin film solar cells

This work explores in situ fabrication of thin film CdS (100 nm)/CdTe (1.5 μm) solar cells using pulsed laser deposition (PLD). Optimization of the PLD processing conditions, including laser energy density, substrate temperature, and the PLD chamber pressure, was achieved with respect to pinhole-free CdS and CdTe layers and solar power conversion efficiency. High efficiency up to 6.68% has been demonstrated and better performance is anticipated with optimization of the PLD process.

Doped graphene nanohole arrays for flexible transparent conductors

Graphene nanohole arrays (GNAs) were fabricated using nanoimprint lithography. The improved optical transmittance of GNAs is primarily due to the reduced surface coverage of graphene from the nanohole fabrication. Importantly, the exposed edges of the nanoholes provided effective sites for chemical doping using thionyl chloride was shown to enhance the conductance by a factor of 15–18 in contrast to only 2-4 for unpatterned graphene. GNAs can provide a unique scheme for improving both optical transmittance and electrical conductivity of graphene-based transparent conductors.

Effects of thermal annealing on noise property and temperature coefficient of resistance of single-walled carbon nanotube films

The effect of thermal annealing on the electrical transport properties of purified and COOH-functionalized single-walled carbon nanotube (SWCNT) films has been investigated and the correlation between the noise property and temperature coefficient of resistance (TCR) has been derived. Thermal annealing has been found highly efficient to improve both noise and TCR properties of the SWCNT films, which is important to applications of SWCNT bolometers. While the improvement may be attributed mainly to the enhanced intertube coupling in the purified SWCNT films, a combined change in both intratube and intertube charge transport is responsible in the case of COOH-functionalized SWCNT films.

Graphene/GaSe-Nanosheet Hybrid: Towards High Gain and Fast Photoresponse

While high photoconductive gain has been recently achieved in graphene-based hybrid phototransistors using semiconductor two-dimensional transition/post-transition metal dichalcogenides or quantum dots sensitizers, obtaining fast photoresponse simutaneously remains a challenge that must be addressed for practical applications. In this paper we report a graphene/GaSe nanosheets hybrid photodetector, in which GaSe nanosheets provide a favorable geometric link to graphene conductive layer through van Der Waals force. After a vacuum annealing process, a high gain in exceeding 107 has been obtained simitaneously with a dynamic response time of around 10 ms for both light on and off. We attribute the high performance to the elimination of possible deep charge traps, most probably at the graphene/GaSe nanosheets interface. This result demonstrates high photoconductive gain and fast photoresponse can be achieved simultaneously and a clean interface is the key to the high performance of these hybrid devices.

High photoresponse in hybrid graphene-carbon nanotube infrared detectors.

Efficient exciton dissociation is crucial to obtaining high photonic response in photodetectors. This work explores implementation of a novel exciton dissociation mechanism through heterojunctions self-assembled at the graphene/MWCNT (multiwall carbon nanotube) interfaces in graphene/MWCNT nanohybrids. Significantly enhanced near-infrared photoresponsivity by nearly an order of magnitude has been achieved on the graphene/MWCNT nanohybrids as compared to the best achieved so far on carbon nanotube (CNT) only infrared (IR) detectors. This leads to a high detectivity up to 1.5 × 10(7) cm·Hz(1/2)·W(-1) in the graphene/MWCNT nanohybrid, which represents a 500% improvement over the best D* achieved on MWCNT film IR detectors and may be further improved with optimization on the interfacial heterojunctions. This approach of the self-assembly of graphene/CNT nanohybrids provides a pathway toward high-performance and low-cost carbon nanostructure IR detectors.

A comparative study of 1/f noise and temperature coefficient of resistance in multiwall and single-wall carbon nanotube bolometers

The 1/f noise and temperature coefficient of resistance (TCR) are investigated in multiwall carbon nanotube (MWCNT) film bolometers since both affect the bolometer detectivity directly. A comparison is made between the MWCNT film bolometers and their single-wall carbon nanotube (SWCNT) counterparts. The intrinsic noise level in the former has been found at least two orders of magnitude lower than that in the latter, which outweighs the moderately lower TCR absolute values in the former and results in higher bolometer detectivity in MWCNT bolometers. Interestingly, reduced noise and enhanced TCR can be obtained by improving the inter-tube coupling using thermal annealing in both SWCNT and MWCNT films, suggesting much higher detectivity may be achieved via engineering the inter-tube coupling.