Semiconductor nanocrystals are promising for use in cheap and highly efficient solar cells. A high efficiency can be achieved by carrier multiplication (CM), which yields multiple electron-hole pairs for a single absorbed photon. Lead chalcogenide nanocrystals are of specific interest, since their band gap can be tuned to be optimal to exploit CM in solar cells. Interestingly, for a given photon energy CM is more efficient in bulk PbS and PbSe, which has been attributed to the higher density of states. Unfortunately, these bulk materials are not useful for solar cells due to their low band gap. Here we demonstrate that two-dimensional PbS nanosheets combine the band gap of a confined system with the high CM efficiency of bulk. Interestingly, in thin PbS nanosheets virtually the entire excess photon energy above the CM threshold is used for CM, in contrast to quantum dots, nanorods and bulk lead chalcogenide materials.

Two-dimensional, solution-processable semiconductor materials are anticipated to be used in low-cost electronic applications, such as transistors and solar cells. Here, lead sulfide nanosheets with a lateral size of several micrometers are synthesized and it is shown how their height can be tuned by the variation of the ligand concentrations. As a consequence of the adjustability of the nanosheets' height between 4 to more than 20 nm charge carriers are in confinement, which has a decisive impact on their electronic properties. This is demonstrated by their use as conduction channel in a field-effect transistor. The experiments show that the performance in terms of current, On/Off ratio, and sub-threshold swing is tunable over a large range.

Two-dimensional materials are considered for future quantum devices and are usually produced by extensive methods like molecular beam epitaxy. We report on the fabrication of field-effect transistors using individual ultra-thin lead sulfide nanostructures with lateral dimensions in the micrometer range and a height of a few nanometers as conductive channel produced by a comparatively fast, inexpensive, and scalable colloidal chemistry approach. Contacted with gold electrodes, such devices exhibit p-type behavior and temperature-dependent photoconductivity. Trap states play a crucial role in the conduction mechanism. The performance of the transistors is among the ones of the best devices based on colloidal nanostructures.Inexpensive electronic applications require semiconductor materials which can be easily processed, e.g. by spin-coating or dip-coating [1]. Thus, researchers are looking for materials that are solution processable while exhibiting reasonable electronic properties. Colloidal semiconductor nanoparticles are among the candidates to be integrated into low-cost electronic devices [2]. They are suspended in liquid media, mass-producible, and tunable in their optical and electrical properties due to quantum confinement effects [3]. Colloidal nanomaterials are promising due to the simplicity and thus the inexpensiveness of their production and subsequent processing. One hurtle which needs to be overcome is the presence of tunnel barriers in the nanoparticle films which lead to high resistances. This effect is the consequence of long isolating organic ligands capping the nanoparticles surface. These ligands can be either replaced by shorter ones including halides [4], or removed by physicochemical processes [5]. These post-treatments deteriorate the nanoparticle surface but reduce the resistive power losses. A different approach to reduce the tunnel barriers consists in the use of inorganic capping "ligands" such as In 2 Se 2− on CdSe nanoparticles [6]. Such films find applications e.g. as field-effect transistors [7], thermoelectrics [8], and photoconductors [9].Yet another approach is to avoid tunnel barrier from the beginning on and to synthesize continuous twodimensional materials in solution. Indeed, some progress has been made in controlling the lateral dimensions [10] and thickness [11] of nanostructures through varying parameters like the nature of the ligands which are used to bind to specific facets of the nanocrystals and inhibit an isotropic growth [12]. Recently, we demonstrated that two-dimensional PbS nanosheets can be produced in solution by colloidal chemistry [13]. We showed that lead sulfide nanosheets form due to two-dimensional oriented attachment of small zero-dimensional colloidal nanocrystals. The nanosheets have a height of a few nanometers and exhibit lateral dimensions in the order of a micron. Nevertheless, the control of anisotropic growth in nanocrystal syntheses is still a great challenge. The PbS nanosheets used in the here presented study were synthesized based on the rec...

Controlling anisotropy in nanostructures is

Solution-processable, two-dimensional semiconductors are promising optoelectronic materials which could find application in low-cost solar cells. Lead sulfide nanocrystals raised attention since the effective band gap can be adapted over a wide range by electronic confinement and observed multi-exciton generation promises higher efficiencies. We report on the influence of the contact metal work function on the properties of transistors based on individual two-dimensional lead sulfide nanosheets. Using palladium we observed mobilities of up to 31 cm(2) V(-1) s(-1). Furthermore, we demonstrate that asymmetrically contacted nanosheets show photovoltaic effect and that the nanosheets' height has a decisive impact on the device performance. Nanosheets with a thickness of 5.4 nm contacted with platinum and titanium show a power conversion efficiency of up to 0.94% (EQE 75.70%). The results underline the high hopes put on such materials.

Employing the spin degree of freedom of charge carriers offers the possibility to extend the functionality of conventional electronic devices, while colloidal chemistry can be used to synthesize inexpensive and tunable nanomaterials. Here, in order to benefit from both concepts, we investigate Rashba spin–orbit interaction in colloidal lead sulphide nanosheets by electrical measurements on the circular photo-galvanic effect. Lead sulphide nanosheets possess rock salt crystal structure, which is centrosymmetric. The symmetry can be broken by quantum confinement, asymmetric vertical interfaces and a gate electric field leading to Rashba-type band splitting in momentum space at the M points, which results in an unconventional selection mechanism for the excitation of the carriers. The effect, which is supported by simulations of the band structure using density functional theory, can be tuned by the gate electric field and by the thickness of the sheets. Spin-related electrical transport phenomena in colloidal materials open a promising pathway towards future inexpensive spintronic devices.

Abstract:Colloidal lead sulfide is a versatile material with great opportunities to tune the bandgap by electronic confinement and to adapt the optical and electrical properties to the target application. We present a new and simple synthetic route to control size and shape of PbS nanoparticles. Increasing concentrations of explicitly added acetic acid are used to tune the shape of PbS nanoparticles from quasi-spherical particles via octahedrons to six-armed stars. The presence of acetate changes the intrinsic surface energies of the different crystal facets and enables the growth along the ⟨100⟩ direction. Furthermore, the presence of 1,2-dichloroethane alters the reaction kinetics, which results in smaller nanoparticles with a narrower size distribution.

Abstract