Search for: All records

Abstract Colloidal quantum dots (QDs)/graphene nanohybrids provide a unique platform to design photodetectors of high performance. These photodetectors are quantum sensors due to the strong quantum confinement in QDs for spectral tunability, and in graphene for high charge mobility. Quantitatively, the high carrier mobility of graphene plays a critical role to enable high photoconductive gain and understanding its impact on the photodetector performance is imperative. Herein, we report a comparative study of PbS QDs/graphene nanohybrids with monolayer and bilayer graphene for broadband photodetection ranging from ultraviolet, visible, near-infrared to short-wave infrared spectra (wavelength: 400 nm–1750 nm) to determine if a specific advantage exists for one over the other. This study has revealed that both the monolayer and bilayer graphene grown in chemical vapor deposition can provide a highly efficient charge transfer channel for photo-generated carriers for high broadband photoresponse. 

Nanohybrids represent a larger variety of functional materials consisting of one or more types of low‐dimensional semiconductor nanostructures, such as quantum dots, nanowires, nanotubes, 2D atomic materials (graphene, transition‐metal dichalcogenides, etc.) interfaced with one another, and/or with conventional material matrices (bulks, films, polymers, etc.). Heterojunction interfaces are characteristic in nanohybrids and play a critical role facilitating synergistic coupling of constituent materials of different functionalities, resulting in excellent electronic, optoelectronic, and mechanical properties. Therefore, nanohybrids provide fresh opportunities for designs of optoelectronic devices of extraordinary performance in addition to the benefits of low cost, large abundance, flexibility, and light weight. Herein, some recent achievements in exploiting new optoelectronic nanohybrids and understanding the underlying physics toward high‐performance optoelectronic nanohybrids that are competitive in commercialization of various optoelectronic devices are highlighted. Using nanohybrid photodetectors as an example, the importance in controlling the heterojunction interfaces and multiscale controlling of optoelectronic process of light absorption, exciton dissociation, photocarrier transfer, and transport from atomic to device scales and how this control impacts the photodetector performance are revealed. The current status, remaining challenges, and future perspectives in optoelectronic nanohybrids are also discussed. 

Magnetic tunnel junctions (MTJs), formed through sandwiching an ultrathin insulating film (so-called tunnel barrier or TB), with ferromagnetic metal electrodes, are fundamental building blocks in magnetoresistive random access memory (MRAM), spintronics, etc. The current MTJ technology employs physical vapor deposition (PVD) to fabricate either amorphous AlOx or epitaxial MgO TBs of thickness around 1 nm or larger to avoid leakage caused by defects in TBs. Motivated by the fundamental limitation in PVD in, and the need for atomically thin and defect-free TBs in MTJs, this work explores atomic layer deposition (ALD) of 1-6 Å thick Al2O3 TBs both directly on Fe films and with an ultrathin Al wetting layer. In situ characterization of the ALD Al2O3 TB was carried out using scanning tunneling spectroscopy (STS). Despite a moderate decrease in TB height Eb with reducing Al wetting layer thicknesses, a remarkable Eb of ∼1.25 eV was obtained on 1 Å thick ALD Al2O3 TB grown directly on an Fe electrode, which is more than twice of that of thermal AlOx TB (∼0.6 eV). Achieving such an atomically thin low-defect TB represents a major step towards improving spin current tunneling in MTJs. 

As metal/insulator/metal tunnel junctions (MIMTJs), such as magnetic tunnel junctions and Josephson tunnel junctions, push the insulating tunnel barrier (TB) towards the ultrathin regime (<1 nm) defects inherent in current physical vapor deposition methods become a fundamental obstacle to create pinhole-free and defect-free MIMTJs. Atomic layer deposition (ALD) could offer a solution by providing a conformal, leak-free tunnel barrier with low defect density and atomic thickness as demonstrated recently in ALD Al2O3 tunnel barriers. A question arises on the viability of the ALD TBs in practical circuits of multilayer structures on which increased roughness may occur. To answer this question, this work investigates electron tunneling properties of ALD Al2O3 tunnel barriers of 1.1 –1.2 Å in thickness on half-cell MIMTJs of Al/Fe/Nb fabricated on multilayer structures of different surface roughness using in situ scanning tunneling spectroscopy. Remarkably, the tunnel barriers grown on the raised multilayer device analogue only show a moderate decrease in barrier height from 1.63 eV, to 1.51 eV and to 1.27 eV as the surface roughness increases from 0.9 nm to 2.3 nm, and to 15 nm, alongside a slight decrease in ALD coverage from ∼96%, to ∼93% and 84% on these samples. Overall, these results validate the ALD TBs of atomic thickness for future 3D arrays of devices. 

Abstract Instability of colloidal iodine‐based inorganic perovskite CsPbX₃(X = Cl, Br, I) nanocrystals (IPNCs) represents a major obstacle in lead‐halide IPNC research and application. Herein, a ligand‐anchoring process is reported that enables significantly improved colloidal stability of the iodine‐based IPNCs for over 10 months in ambient. Apart from the previous efforts in searching for strong binding ligands to cap the IPNCs to incrementally reduce the exposure of the IPNC surface to the harsh colloidal environment, the ligand‐anchoring method demonstrates that such an exposure can be reduced substantially by suppressing the dynamic ligand exchange around the colloidal IPNCs. In the IPNC synthesis solution with common oleic acid (OA) and oleylamine (OLA) ligands with relative weak binding to IPNCs, a systematic reduction of the ligand concentration using hexane by an order of magnitude has shown to be effective in achieving OA/OLA ligand‐anchored iodine‐based IPNCs with superior stability as confirmed in optical absorption, photoluminescence,¹H solution nuclear magnetic resonance spectroscopy, and photoresponse. This result has revealed that the intermittent exposure of the IPNC surface during the dynamic ligand exchange is a primary mechanism underlying the colloidal IPNC instability, which can be resolved in the ligand‐anchoring process by suppressing such dynamic activities. 

Abstract Localized surface plasmon resonance (LSPR) is shown to be effective in trapping light for enhanced light absorption and hence performance in photonic and optoelectronic devices. Implementation of LSPR in all‐inorganic perovskite nanocrystals (PNCs) is particularly important considering their unique advantages in optoelectronics. Motivated by this, the first success in colloidal synthesis of AuCu/CsPbCl₃core/shell PNCs and observation of enhanced light absorption by the perovskite CsPbCl₃shell of thickness in the range of 2–4 nm, enabled by the LSPR AuCu core of an average diameter of 7.1 nm, is reported. This enhanced light absorption leads to a remarkably enhanced photoresponse in PNCs/graphene nanohybrid photodetectors using the AuCu/CsPbCl₃core/shell PNCs, by more than 30 times as compared to the counterparts with CsPbCl₃PNCs only (8–12 nm in dimension). This result illustrates the feasibility in implementation of LSPR light trapping directly in core/shell PNCs for high‐performance optoelectronics. 

Abstract Cation–π interactions between molecules and graphene are known to have a profound effect on the properties of the molecule/graphene nanohybrids and motivate this study to quantify the attachment of the rhodamine 6G (R6G) dye molecules on graphene and the photocarrier transfer channel formed across the R6G/graphene interface. By increasing the R6G areal density of the R6G on graphene field‐effect transistor (GFET) from 0 up to ≈3.6 × 10¹³cm⁻², a linear shift of the Dirac point of the graphene from originally 1.2 V (p‐doped) to −1 V (n‐doped) is revealed with increasing number of R6G molecules. This indicates that the attachment of the R6G molecules on graphene is determined by the cation–π interaction between the NH+ in R6G and π electrons in graphene. Furthermore, a linear dependence of the photoresponse on the R6G molecule concentration to 550 nm illumination is observed on the R6G/graphene nanohybrid, suggesting that the cation–π interaction controls the R6G attachment configuration to graphene to allow formation of identical photocarrier transfer channels. On R6G/graphene nanohybrid with 7.2 × 10⁷R6G molecules, high responsivity up to 5.15 × 10²A W⁻¹is obtained, suggesting molecule/graphene nanohybrids are promising for high‐performance optoelectronics. 

Abstract Two‐dimensional transition metal dichalcogenides (TMDs)/graphene van der Waals (vdW) heterostructures integrate the superior light–solid interaction in TMDs and charge mobility in graphene, and therefore are promising for surface‐enhanced Raman spectroscopy (SERS). Herein, a novel TMD (MoS₂and WS₂) nanodome/graphene vdW heterostructure SERS substrate, on which an extraordinary SERS sensitivity is achieved, is reported. Using fluorescent Rhodamine 6G (R6G) as probe molecules, the SERS sensitivity is in the range of 10⁻¹¹to 10⁻¹²mon the TMD nanodomes/graphene vdW heterostructure substrates using 532 nm Raman excitation, which is comparable to the best sensitivity reported so far using plasmonic metal nanostructures/graphene SERS substrates, and is more than three orders of magnitude higher than that on single‐layer TMD and graphene substrates. Density functional theory simulation reveals enhanced electric dipole moments and dipole–dipole interaction at the TMD/graphene vdW interface, yielding an effective means to facilitate an external electrostatic perturbation on the graphene surface and charge transfer. This not only promotes chemical enhancement on SERS, but also enables electromagnetic enhancement of SERS through the excitation of localized surface plasmonic resonance on the TMD nanodomes. This TMD nanodome/graphene vdW heterostructure is therefore promising for commercial applications in high‐performance optoelectronics and sensing. 

Abstract Lateral p–n junctions take the unique advantages of 2D materials, such as graphene, to enable single‐atomic layer microelectronics. A major challenge in fabrication of the lateral p–n junctions is in the control of electronic properties on a 2D atomic sheet with nanometer precision. Herein, a facile approach that employs decoration of molecular anions of bis‐(trifluoromethylsulfonyl)‐imide (TFSI) to generate p‐doping on the otherwise n‐doped graphene by positively polarized surface electric dipoles (pointing toward the surface) formed on the surface oxygen‐deficient layer “intrinsic” to an oxide ferroelectric back gate is reported. The characteristic double conductance minimaV_Dirac−andV_Dirac+illustrated in the obtained lateral graphene p–n junctions can be tuned in the range of −1 to 0 V and 0 to +1 V, respectively, by controlling the TFSI anions and surface dipoles quantitatively. The unique advantage of this approach is in adoption of polarity‐controlled molecular ion attachment on graphene, which could be further developed for various lateral electronics on 2D materials. 

Abstract Van der Waals (vdW) heterostructures of 2D atomically thin layered materials (2DLMs) provide a unique platform for constructing optoelectronic devices by staking 2D atomic sheets with unprecedented functionality and performance. A particular advantage of these vdW heterostructures is the energy band engineering of 2DLMs to achieve interlayer excitons through type‐II band alignment, enabling spectral range exceeding the cutoff wavelengths of the individual atomic sheets in the 2DLM. Herein, the high performance of GaTe/InSe vdW heterostructures device is reported. Unexpectedly, this GaTe/InSe vdWs p–n junction exhibits extraordinary detectivity in a new shortwave infrared (SWIR) spectrum, which is forbidden by the respective bandgap limits for the constituent GaTe (bandgap of ≈1.70 eV in both the bulk and monolayer) and InSe (bandgap of ≈1.20–1.80 eV depending on thickness reduction from bulk to monolayer). Specifically, the uncooled SWIR detectivity is up to ≈10¹⁴Jones at 1064 nm and ≈10¹²Jones at 1550 nm, respectively. This result indicates that the 2DLM vdW heterostructures with type‐II band alignment produce an interlayer exciton transition, and this advantage can offer a viable strategy for devising high‐performance optoelectronics in SWIR or even longer wavelengths beyond the individual limitations of the bandgaps and heteroepitaxy of the constituent atomic layers.