Rationalizing energy level alignment by characterizing Lewis acid/base and ionic interactions at printable semiconductor/ionic liquid interfaces.

Charge transfer and energy conversion processes at semiconductor/electrolyte interfaces are controlled by local electric field distributions, which can be especially challenging to measure. Herein we leverage the low vapor pressure and vacuum compatibility of ionic liquid electrolytes to undertake a layer-by-layer, ultra-high vacuum deposition of a prototypical ionic liquid EMIM+ (1-ethyl-3-methylimidazolium) and TFSI- (bis(trifluoromethylsulfonyl)-imide) on the surfaces of different electronic materials. We consider a case-by-case study between a standard metal (Au) and four printed electronic materials, where interfaces are characterized by a combination of X-ray and ultraviolet photoemission spectroscopies (XPS/UPS). For template-stripped gold surfaces, we observe through XPS a preferential orientation of the TFSI anion at the gold surface, enabling large electric fields (∼108 eV m-1) within the first two monolayers detected by a large surface vacuum level shift (0.7 eV) in UPS. Conversely, we observe a much more random orientation on four printable semiconductor surfaces: methyl ammonium lead triiodide (MAPbI3), regioregular poly(3-hexylthiophene-2,5-diyl (P3HT)), sol-gel nickel oxide (NiOx), and PbIx-capped PbS quantum dots. For the semiconductors considered, the ionization energy (IE) of the ionic liquid at 3 ML coverage is highly substrate dependent, indicating that underlying chemical reactions are dominating interface level alignment (electronic equilibration) prior to reaching bulk electronic structure. This indicates there is no universal rule for energy level alignment, but that relative strengths of Lewis acid/base sites and ion-molecular interactions should be considered. Specifically, for P3HT, interactions are found to be relatively weak and occurring through the π-bonding structure in the thiophene ring. Alternatively, for NiOx, PbS/PbIx quantum dots, and MAPbI3, our XPS data suggest a combination of ionic bonding and Lewis acid/base reactions between the semiconductor and IL, with MAPbI3 being the most reactive surface. Collectively, our results point towards new directions in interface engineering, where strategically chosen ionic liquid-based anions and cations can be used to preferentially passivate and/or titrate surface defects of heterogeneous surfaces while simultaneously providing highly localized electric fields. These opportunities are expected to be translatable to opto-electronic and electrochemical devices, including energy conversion and storage and biosensing applications.

Enhanced Infrared Photodiodes Based on PbS/PbClx Core/Shell Nanocrystals.

Improved passivation strategies to address the more complex surface structure of large-diameter nanocrystals are critical to the advancement of infrared photodetectors based on colloidal PbS. In this contribution, the performance of short-wave infrared (SWIR) photodiodes fabricated with PbS/PbClx (core/shell) nanocrystals vs their PbS-only (core) counterparts are directly compared. Devices using PbS cores suffer from shunting and inefficient charge extraction, while core/shell-based devices exhibit greater external quantum efficiencies and lower dark current densities. To elucidate the implications of the shell chemistry on device performance, thickness-dependent energy level offsets and interfacial chemistry of nanocrystal films with the zinc oxide electron-transport layer are evaluated. The disparate device performance between the two synthetic methods is attributed to unfavorable interface dipole formation and surface defect states, associated with inadequate removal of native organic ligands in core-only films. The core/shell system offers a promising route to manage the additional nonpolar (100) surface facets of larger nanocrystals that conventional halide ligand treatments fail to sufficiently passivate.

Binary Superlattices of Infrared Plasmonic and Excitonic Nanocrystals.

Self-assembled superlattices of nanocrystals offer exceptional control over the coupling between nanocrystals, similar to how solid-state crystals tailor the bonding between atoms. By assembling nanocrystals of different properties (e.g., plasmonic, excitonic, dielectric, or magnetic), we can form a wealth of binary superlattice metamaterials with new functionalities. Here, we introduce infrared plasmonic Cu2-xS nanocrystals to the limited library of materials that have been successfully incorporated into binary superlattices. We are the first to create a variety of binary superlattices with large excitonic (PbS) nanocrystals and small plasmonic (Cu2-xS) nanocrystals, both resonant in the infrared. Then, by controlling the surface chemistry of large Cu2-xS nanocrystals, we produced structurally analogous superlattices of large Cu2-xS and small PbS nanocrystals. Transmission electron microscopy (TEM) and grazing-incidence small-angle X-ray scattering (GISAXS) were used to characterize both types of superlattices. Furthermore, our unique surface modification of the large Cu2-xS nanocrystals also prevented them from chemically quenching the photoluminescence of the PbS nanocrystals, which occurred when the PbS nanocrystals were mixed with unmodified Cu2-xS nanocrystals. These synthetic achievements create a set of binary superlattices that can be used to understand how infrared plasmonic and excitonic nanocrystals couple in a variety of symmetries and stoichiometries.

Controlling dissolution of PbTe nanoparticles in organic solvents during liquid cell transmission electron microscopy.

We present direct visualization of the dynamics of oleic-acid-capped PbTe nanoparticles suspended in different organic solvents using liquid cell transmission electron microscopy. Liquid cell transmission electron microscopy is a powerful tool to directly observe the behavior of a variety of nanoparticles in liquids, but requires careful consideration and quantification of how the electron beam affects the systems being investigated. We find that etching and dissolution of PbTe nanoparticles occurs with a strong dependence on electron dose rate ranging from no perceivable effect on the nanoparticles with lower dose rates (50 e- Å-2 s-1) to complete dissolution within seconds or minutes at higher dose rates (100 and 200 e- Å-2 s-1). We propose that oxidative etching, resulting from the radiolysis of small amounts of water, causes the PbTe nanoparticles to dissolve after exposure to a threshold electron dose rate of 50 e- Å-2 s-1.

Effects of a Lead Chloride Shell on Lead Sulfide Quantum Dots.

The size of a quantum-confined nanocrystal determines the energies of its excitonic transitions. Previous work has correlated the diameters of PbS nanocrystals to their excitonic absorption; however, we observe that PbS quantum dots synthesized in saturated dispersions of PbCl2 can deviate from the previous 1Sh-1Se energy vs diameter curve by 0.8 nm. In addition, their surface differs chemically from that of PbS quantum dots produced via other syntheses. We find that these nanocrystals are coated in a shell that is measurable in transmission electron micrographs and contains lead and chlorine, beyond the monatomic chlorine termination previously proposed. This finding has implications for understanding the growth mechanism of this reaction, the line width of these quantum dots' photoluminescence, and electronic transport within films of these nanocrystals. Such fundamental knowledge is critical to applications of PbS quantum dots such as single-photon sources, photodetectors, solar cells, light-emitting diodes, lasers, and biological labels.

Synthesis and Optical Properties of PbSe Nanorods with Controlled Diameter and Length.

The synthesis of PbSe nanorods with low branching (<1%), high aspect ratios (up to ∼16), and controlled lengths and diameters was demonstrated via the removal of water and oleic acid from the synthesis precursors. It was determined that the proper combination of reaction time and temperature allows for the control of PbSe nanorod length and diameter and therefore control over their electronic states, as probed through absorbance and photoluminescence measurements. Similar to PbSe nanowires, nanorods display higher Stokes shifts than for spherical nanocrystals due to intrananorod diameter fluctuations.

Anisotropic absorption in PbSe nanorods.

We present absorption anisotropy measurements in PbSe nanostructures. This is accomplished via a new means of measuring absorption anisotropy in randomly oriented solution ensembles of nanostructures via pump-probe spectroscopy, which exploits the polarization memory effect. We observe isotropic absorption in nanocrystals and anisotropic absorption in nanorods, which increases upon elongation from aspect ratio 1 to 4 and is constant for longer nanorods. The measured volume-normalized absorption cross section is 1.8 ± 0.3 times larger for parallel pump and probe polarizations in randomly oriented nanorods as compared to nanocrystals. We show that this enhancement would be larger than an order of magnitude for aligned nanorods. Despite being in the strong quantum confinement regime, the aspect ratio dependence of the absorption anisotropy in PbSe nanorods is described classically by the effects of dielectric contrast on an anisotropic nanostructure. These results imply that the dielectric constant of the surrounding medium can be used to influence the optoelectronic properties of nanorods, including polarized absorption and emission, phonon modes, multiple exciton generation efficiency, and Auger recombination rate.

Control of PbSe nanorod aspect ratio by limiting phosphine hydrolysis.

The aspect ratio and yield of PbSe nanorods synthesized by the reaction of Pb-oleate with tris(diethylamino)phosphine selenide are highly sensitive to the presence of water, making it critical to control the amount of water present in the reaction. By carefully drying the reaction precursors and then intentionally adding water back into the reaction, the nanorod aspect ratio can be controlled from 1.1 to 10 and the yield from 1 to 14% by varying the water concentration from 0 to 204 mM. (31)P{(1)H} and (1)H NMR show that water reacts with tris(diethylamino)phosphine to create bis(diethylamido)phosphorous acid. It was determined that bis(diethylamido)phosphorous acid is responsible for the observed aspect ratio and yield changes. Finally, it was found that excess oleic acid in the reaction can also react with tris(diethylamino)phosphine to create bis(diethylamido)phosphorous acid, and upon the removal of both excess oleic acid and water, highly uniform, nonbranching nanorods were formed.

Enhanced open-circuit voltage of PbS nanocrystal quantum dot solar cells.

Nanocrystal quantum dots (QD) show great promise toward improving solar cell efficiencies through the use of quantum confinement to tune absorbance across the solar spectrum and enable multi-exciton generation. Despite this remarkable potential for high photocurrent generation, the achievable open-circuit voltage (Voc) is fundamentally limited due to non-radiative recombination processes in QD solar cells. Here we report the highest open-circuit voltages to date for colloidal QD based solar cells under one sun illumination. This Voc of 692 ± 7 mV for 1.4 eV PbS QDs is a result of improved passivation of the defective QD surface, demonstrating Voc(mV)=553Eg/q-59 as a function of the QD bandgap (Eg). Comparing experimental Voc variation with the theoretical upper-limit obtained from one diode modeling of the cells with different Eg, these results clearly demonstrate that there is a tremendous opportunity for improvement of Voc to values greater than 1 V by using smaller QDs in QD solar cells.

Enhanced multiple exciton generation in quasi-one-dimensional semiconductors.

The creation of a single electron-hole pair (i.e., exciton) per incident photon is a fundamental limitation for current optoelectronic devices including photodetectors and photovoltaic cells. The prospect of multiple exciton generation per incident photon is of great interest to fundamental science and the improvement of solar cell technology. Multiple exciton generation is known to occur in semiconductor nanostructures with increased efficiency and reduced threshold energy compared to their bulk counterparts. Here we report a significant enhancement of multiple exciton generation in PbSe quasi-one-dimensional semiconductors (nanorods) over zero-dimensional nanostructures (nanocrystals), characterized by a 2-fold increase in efficiency and reduction of the threshold energy to (2.23 ± 0.03)E(g), which approaches the theoretical limit of 2E(g). Photovoltaic cells based on PbSe nanorods are capable of improved power conversion efficiencies, in particular when operated in conjunction with solar concentrators.

Photosensitization of ZnO nanowires with CdSe quantum dots for photovoltaic devices.

We combine CdSe semiconductor nanocrystals (or quantum dots) and single-crystal ZnO nanowires to demonstrate a new type of quantum-dot-sensitized solar cell. An array of ZnO nanowires was grown vertically from a fluorine-doped tin oxide conducting substrate. CdSe quantum dots, capped with mercaptopropionic acid, were attached to the surface of the nanowires. When illuminated with visible light, the excited CdSe quantum dots injected electrons across the quantum dot-nanowire interface. The morphology of the nanowires then provided the photoinjected electrons with a direct electrical pathway to the photoanode. With a liquid electrolyte as the hole transport medium, quantum-dot-sensitized nanowire solar cells exhibited short-circuit currents ranging from 1 to 2 mA/cm2 and open-circuit voltages of 0.5-0.6 V when illuminated with 100 mW/cm2 simulated AM1.5 spectrum. Internal quantum efficiencies as high as 50-60% were also obtained.