Stabilization of Colloidal Germanium Nanoparticles: From the Study to the Prospects of the Application in Thin-Film Technology.

We present the stabilization of halide-terminated Ge nanoparticles prepared via a disproportionation reaction of metastable Ge(I)X solutions with well-defined size distribution. Further tailoring of the stability of the Ge nanoparticles was achieved using variations in the substituent. Ge nanoparticles obtained in this way are readily dispersed in organic solvents, long-term colloidally stable, and are perfect prerequisites for thin-film preparation. This gives these nanomaterials a future in surface-dependent optical applications, as shown for the halide-terminated nanoparticles.

Edge contacts accelerate the response of MoS2 photodetectors.

We use a facile plasma etching process to define contacts with an embedded edge geometry for multilayer MoS2 photodetectors. Compared to the conventional top contact geometry, the detector response time is accelerated by more than an order of magnitude by this action. We attribute this improvement to the higher in-plane mobility and direct contacting of the individual MoS2 layers in the edge geometry. With this method, we demonstrate electrical 3 dB bandwidths of up to 18 MHz which is one of the highest values reported for pure MoS2 photodetectors. We anticipate that this approach should also be applicable to other layered materials, guiding a way to faster next-generation photodetectors.

Heading toward Miniature Sensors: Electrical Conductance of Linearly Assembled Gold Nanorods.

Metal nanoparticles are increasingly used as key elements in the fabrication and processing of advanced electronic systems and devices. For future device integration, their charge transport properties are essential. This has been exploited, e.g., in the development of gold-nanoparticle-based conductive inks and chemiresistive sensors. Colloidal wires and metal nanoparticle lines can also be used as interconnection structures to build directional electrical circuits, e.g., for signal transduction. Our scalable bottom-up, template-assisted self-assembly creates gold-nanorod (AuNR) lines that feature comparably small widths, as well as good conductivity. However, the bottom-up approach poses the question about the consistency of charge transport properties between individual lines, as this approach leads to heterogeneities among those lines with regard to AuNR orientation, as well as line defects. Therefore, we test the conductance of the AuNR lines and identify requirements for a reliable performance. We reveal that multiple parallel AuNR lines (>11) are necessary to achieve predictable conductivity properties, defining the level of miniaturization possible in such a setup. With this system, even an active area of only 16 µm2 shows a higher conductance (~10-5 S) than a monolayer of gold nanospheres with dithiolated-conjugated ligands and additionally features the advantage of anisotropic conductance.

Substrate effects on the speed limiting factor of WSe2 photodetectors.

We investigate the time-resolved photoelectric response of WSe2 crystals on glass and flexible polyimide substrates to determine the effect of a changed dielectric environment on the speed of the photodetectors. We show that varying the substrate material can alter the speed-limiting mechanism: while the detectors on polyimide are RC limited, those on glass are limited by slower excitonic diffusion processes. We attribute this to a shortening of the depletion layer at the metal electrode/WSe2 interface caused by the higher dielectric screening of glass compared to polyimide. The photodetectors on glass show a tunable bandwidth, which can be increased to 2.6 MHz with increasing the electric field.

Sub-nanosecond Intrinsic Response Time of PbS Nanocrystal IR-Photodetectors.

Colloidal nanocrystals (NCs), especially lead sulfide NCs, are promising candidates for solution-processed next-generation photodetectors with high-speed operation frequencies. However, the intrinsic response time of PbS-NC photodetectors, which is the material-specific physical limit, is still elusive, as the reported response times are typically limited by the device geometry. Here, we use the two-pulse coincidence photoresponse technique to identify the intrinsic response time of 1,2-ethanedithiol-functionalized PbS-NC photodetectors after femtosecond-pulsed 1560 nm excitation. We obtain an intrinsic response time of ∼1 ns, indicating an intrinsic bandwidth of ∼0.55 GHz as the material-specific limit. Examination of the dependence on laser power, gating, bias, temperature, channel length, and environmental conditions suggest that Auger recombination, assisted by NC-surface defects, is the dominant mechanism. Accordingly, the intrinsic response time might further be tuned by specifically controlling the ligand coverage and trap states. Thus, PbS-NC photodetectors are feasible for gigahertz optical communication in the third telecommunication window.

Substrate Effects on the Bandwidth of CdSe Quantum Dot Photodetectors.

We investigate the time-resolved photocurrent response of CdSe quantum dot (QD) thin films sensitized with zinc ß-tetraaminophthalocyanine (Zn4APc) (Kumar , ACS Appl. Mater. Interfaces, 2019, 11, 48271-48280) on three different substrates, namely, silicon with 230 nm SiO2 dielectric, glass, and polyimide. While Si/SiO2 (230 nm) is not suitable for any transient photocurrent characterization due to an interfering photocurrent response of the buried silicon, we find that polyimide substrates invoke the larger optical bandwidth with 85 kHz vs 67 kHz for the same quantum dot thin film on glass. Upon evaluation of the transient photocurrent, we find that the photoresponse of the CdSe quantum dot films can be described as a combination of carrier recombination and fast trapping within 2.7 ns followed by slower multiple trapping events. The latter are less pronounced on polyimide, which leads to the higher bandwidth. We show that all devices are resistance-capacitance (RC)-time limited and that improvements of photoresistance are the key to further increasing the bandwidth.

Fast, Infrared-Active Optical Transistors Based on Dye-Sensitized CdSe Nanocrystals.

We report an optically gated transistor composed of CdSe nanocrystals (NCs), sensitized with the dye zinc ß-tetraaminophthalocyanine for operation in the first telecom window. This device shows a high ON/OFF ratio of 6 orders of magnitude in the red spectral region and an unprecedented 4.5 orders of magnitude at 847 nm. By transient absorption spectroscopy, we reveal that this unexpected infrared sensitivity is due to electron transfer from the dye to the CdSe NCs within 5 ps. We show by time-resolved photocurrent measurements that this enables fast rise times during near-infrared optical gating of 47 ± 11 ns. Electronic coupling and accelerated nonradiative recombination of charge carriers at the interface between the dye and the CdSe NCs are further corroborated by steady-state and time-resolved photoluminescence measurements. Field-effect transistor measurements indicate that the increase in photocurrent upon laser illumination is mainly due to the increase in the carrier concentration while the mobility remains unchanged. Our results illustrate that organic dyes as ligands for NCs invoke new optoelectronic functionalities, such as fast optical gating at sub-bandgap optical excitation energies.

Interface properties of CoPc and CoPcF16 on graphene/nickel: influence of germanium intercalation.

Photoelectron spectroscopy was used to investigate electronic interface properties and interactions of the organic semiconductors CoPc and CoPcF16 on graphene/nickel based substrates. Additional focus was put on the influence of germanium intercalation of graphene/nickel. The presented results demonstrate that germanium can decouple graphene from nickel and in this manner restore its buffer layer properties. No charge transfer from the substrate to the organic layer is observed in the germanium intercalated case, while interface related peaks in the Co 2p core level spectra indicate such charge transfer on graphene/nickel. Strong interface dipoles are found for CoPcF16 on graphene/nickel and on germanium intercalated graphene/nickel. Fluorine Auger parameters have been measured, and the results provide evidence for polarization and charge transfer screening effects of different amounts at the unlike film-substrate interfaces. The various contributions to the observed shifts are discussed.