Fabrication Of All-inorganic Nanocrystal Solids Through Matrix Encapsulation Of Nanocrystal Arrays

A general strategy for low-temperature processing of colloidal nanocrystals into all-inorganic films is reported. The present methodology goes beyond the traditional ligand-interlinking scheme and relies on encapsulation of morphologically defined nanocrystal arrays into a matrix of a wide-band gap semiconductor, which preserves optoelectronic properties of individual nanoparticles while rendering the nanocrystal film photoconductive. Fabricated solids exhibit excellent thermal stability, which is attributed to the heteroepitaxial structure of nanocrystal matrix interfaces, and show compelling light-harvesting performance in prototype solar cells

The Role of Hole Localization in Sacrificial Hydrogen Production by Semiconductor-Metal Heterostructured Nanocrystals

The effect of hole localization on photocatalytic activity of Pt-tipped semiconductor nanocrystals is investigated. By tuning the energy balance at the semiconductor-ligand interface, we demonstrate that hydrogen production on Pt sites is efficient only when electron-donating molecules are used for stabilizing semiconductor surfaces. These surfactants play an important role in enabling an efficient and stable reduction of water by heterostructured nanocrystals as they fill vacancies in the valence band of the semiconductor domain, preventing its degradation. In particular, we show that the energy of oxidizing holes can be efficiently transferred to a ligand moiety, leaving the semiconductor domain intact. This allows reusing the inorganic portion of the degraded nanocrystal-ligand system simply by recharging these nanoparticles with fresh ligands

Photocatalytic Activity Of Core/shell Semiconductor Nanocrystals Featuring Spatial Separation Of Charges

The present study investigates the photocatalytic activity of ZnSe/CdS core/shell semiconductor nanocrystals. These nanoparticles exhibit a spatial separation of photoinduced charges between the core and the shell domains, which makes them potentially viable for photocatalytic applications. Unfortunately, one of the excited charges remains inside the core semiconductor and thus cannot efficiently react with the external environment. Here, we explore this issue by investigating the mechanisms of hole extraction from the ZnSe core to the surface of the CdS shell. In particular, the effect of shell thickness in ZnSe/CdS core/shell nanocrystals on the ability of core-localized charges to perform oxidative reactions was determined. By using a combination of time-resolved spectroscopy and electrochemical techniques, we demonstrate that the use of hole-scavenging surfactants facilitates an efficient transfer of core-localized holes to the surface even in the case of shells exceeding 7 nm in thickness. These measurements further demonstrate that photoinduced holes can be extracted from the core faster than they recombine with shell-localized electrons, indicating that most of the absorbed energy in ZnSe/CdS nanocrystals can be used to drive catalytic reactions

Monolayer Solid-State Electrolyte for Electric Double Layer Gating of Graphene Field-Effect Transistors

The electrostatic gating of graphene field-effect transistors is demonstrated using a monolayer electrolyte. The electrolyte, cobalt crown ether phthalocyanine (CoCrPc) and LiClO₄, is deposited as a monolayer on the graphene channel, essentially creating an additional two-dimensional layer on top of graphene. The crown ethers on the CoCrPc solvate lithium ions and the ion location is modulated by a backgate without requiring liquid solvent. Ions dope the channel by inducing image charges; the doping level (<i>i</i>.<i>e</i>., induced charge density) can be modulated by the backgate bias with the extent of the surface potential change being controlled by the magnitude and polarity of the backgate bias. With a crown ether to Li⁺ ratio of 5:1, programming tests for which the backgate is held at −<i>V</i>_BG shift the Dirac point by ∼15 V, corresponding to a sheet carrier density on the order of 10¹² cm^–2. This charge carrier density agrees with the packing density of monolayer CoCrPc on graphene that would be expected with one Li⁺ for every five crown ethers (at the maximum possible Li⁺ concentration, 10¹³ cm^–2 is predicted). The crown ethers provide two stable states for the Li⁺: one near the graphene channel (low-resistance state) and one ∼5 Å away from the channel (high-resistance state). Initial state retention measurements indicate that the two states can be maintained for at least 30 min (maximum time monitored), which is 10⁶ times longer than polymer-based electrolytes at room temperature, with at least a 250 Ω μm difference between the channel resistance in the high- and low-resistance states

Increasing the Room-Temperature Electric Double Layer Retention Time in Two-Dimensional Crystal FETs

Poly­(vinyl alcohol) (PVA) and LiClO₄, a solid polymer electrolyte with a glass transition temperature (<i>T</i>_g) of 80 °C, is used to electrostatically gate graphene field-effect transistors. The ions in PVA:LiClO₄ are drifted into place by field-effect at <i>T</i> > <i>T</i>_g, providing <i>n</i>- or <i>p</i>-type doping, and when the device is cooled to room temperature, the polymer mobility and, hence ion mobility are arrested and the electric double layer (EDL) is “locked” into place in the absence of a gate bias. Unlike other electrolytes used to gate two-dimensional devices for which the <i>T</i>_g, and therefore the “locking” temperature, is well below room temperature, the electrolyte demonstrated in this work provides a route to achieve room-temperature EDL stability. Specifically, a 6 orders of magnitude increase in the room temperature EDL retention time is demonstrated over the commonly used electrolyte, poly­(ethylene oxide) (PEO) and LiClO₄. Hall measurements confirm that large sheet carrier densities can be achieved with PVA:LiClO₄ at top gate programming voltages of ±2 V (−6.3 ± 0.03 × 10¹³ cm^–2 for electrons and 1.6 ± 0.3 × 10¹⁴ cm^–2 for holes). Transient drain current measurements show that at least 75% of the EDL is retained after more than 4 h at room temperature. Unlike PEO-based electrolytes, PVA:LiClO₄ is compatible with the chemicals used in standard photolithographic processes enabling the direct deposition of patterned, metal contacts on the surface of the electrolyte. A thermal instability in the electrolyte is detected by both <i>I</i>–<i>V</i> measurements and differential scanning calorimetry, and FTIR measurements suggest that thermally catalyzed cross-linking may be driving phase separation between the polymer and the salt. Nevertheless, this work highlights how the relationship between polymer and ion mobility can be exploited to tune the state retention time and the charge carrier density of a 2D crystal transistor

Photocatalytic Activity of Core/Shell Semiconductor Nanocrystals Featuring Spatial Separation of Charges

The present study investigates the photocatalytic activity of ZnSe/CdS core/shell semiconductor nanocrystals. These nanoparticles exhibit a spatial separation of photoinduced charges between the core and the shell domains, which makes them potentially viable for photocatalytic applications. Unfortunately, one of the excited charges remains inside the core semiconductor and thus cannot efficiently react with the external environment. Here, we explore this issue by investigating the mechanisms of hole extraction from the ZnSe core to the surface of the CdS shell. In particular, the effect of shell thickness in ZnSe/CdS core/shell nanocrystals on the ability of core-localized charges to perform oxidative reactions was determined. By using a combination of time-resolved spectroscopy and electrochemical techniques, we demonstrate that the use of hole-scavenging surfactants facilitates an efficient transfer of core-localized holes to the surface even in the case of shells exceeding 7 nm in thickness. These measurements further demonstrate that photoinduced holes can be extracted from the core faster than they recombine with shell-localized electrons, indicating that most of the absorbed energy in ZnSe/CdS nanocrystals can be used to drive catalytic reactions