Role of Precursor Reactivity in Crystallization of Solution-Processed Semiconductors: The Case of Cu₂ZnSnS₄

We study the formation of Cu₂ZnSnS₄ (CZTS) films from various liquid-phase precursors. Our experimental data point to the significant role that reactivities of precursor components play in the quality of the final material. Although reactive molecular precursors favor formation of CZTS under milder conditions, the formation of large crystalline domains requires using less reactive nanostructured precursors. We explain this effect using kinetics of nucleation and growth. We have also demonstrated a strategy to effectively enhance grain growth of CZTS using solid-state phase transition as the driving force for nanocrystal sintering. We hope this contribution will provide a useful guide toward the rational design of liquid-phase precursors for inorganic semiconductors for electronic and optoelectronic applications

III–V Nanocrystals Capped with Molecular Metal Chalcogenide Ligands: High Electron Mobility and Ambipolar Photoresponse

In this work, we synthesized InP and InAs nanocrystals (NCs) capped with different inorganic ligands, including various molecular metal chalcogenide complexes (MCCs) and chalcogenide ions. We found that MCCs and chalcogenide ions can quantitatively displace organic ligands from the surface of III–V NCs and serve as the inorganic capping groups for III–V NC surfaces. These inorganic ligands stabilize colloidal solutions of InP and InAs NCs in polar solvents and greatly facilitate charge transport between individual NCs. Charge transport studies revealed high electron mobility in the films of MCC-capped InP and InAs NCs. For example, we found that bridging InAs NCs with Cu₇S₄^– MCC ligands can lead to very high electron mobility exceeding 15 cm²/(V s). In addition, we observed unprecedented ambipolar (positive/negative) photoresponse of MCC-capped InAs NC solids that changed sign depending on the ligand chemistry, illumination wavelength, and doping of the NC solid. For example, the sign of photoconductance of InAs NCs capped with Cu₇S₄^– or Sn₂S₆^4– ions converted from positive at 0.80 and 0.95 eV to negative at 1.27 and 1.91 eV. We propose an explanation of this unusually complex photoconductivity of InAs NC solids

Colloidal Nanocrystals with Inorganic Halide, Pseudohalide, and Halometallate Ligands

We investigate simple halides and pseudohalides as an important class of inorganic ligands for nanocrystals (NCs) in solution phase ligand exchange. These short, robust, and easy to model ligands bind to the NC surface and provide electrostatic stabilization of NC dispersions in <i>N</i>-methylformamide. The replacement of organic ligands on NCs with compact halide and pseudohalide ligands greatly facilitates electronic communication between NCs. For example, a high electron mobility of μ ≈ 12 cm² V^–1 s^–1 has been observed in thin films made of I^–-capped CdSe NCs. We also studied charge transport properties of thin films based on the pseudohalide N₃^–-capped InAs NCs, suggesting the possibility of obtaining “all III–V” NC solids. In addition, we extend the surface chemistry of halometallates (<i>e.g.,</i> CH₃NH₃PbI₃), which can stabilize colloidal solutions of lead chalcogenide NCs. These halide, pseudohalide, and halometallate ligands enrich the current family of inorganic ligands and can open up more opportunities for applications of NCs in the fields of electronics, optoelectronics, and thermoelectrics

Carrier Dynamics in Highly Quantum-Confined, Colloidal Indium Antimonide Nanocrystals

Nanometer-sized particles of indium antimonide (InSb) offer opportunities in areas such as solar energy conversion and single photon sources. Here, we measure electron–hole pair dynamics, spectra, and absorption cross sections of strongly quantum-confined colloidal InSb nanocrystal quantum dots using femtosecond transient absorption. For all samples, we observe a bleach feature that develops on ultrafast time scales, which notably moves to lower energy during the first several picoseconds following excitation. We associate this unusual red shift, which becomes larger for larger particles and more distinct at lower sample temperatures, with hot exciton cooling through states that we suggest arise from energetically proximal conduction band levels. From controlled optical excitation intensities, we determine biexciton lifetimes, which range from 2 to 20 ps for the studied 3–6 nm diameter particle sizes

Thermal Stability of Semiconductor Nanocrystal Solids: Understanding Nanocrystal Sintering and Grain Growth

Nanomaterials are naturally metastable with respect to bulk solids. This raises the very important fundamental problem of their morphological stability, especially when nanoscale crystallites are touching or nearly touching each other, such as in thin-film devices. In some cases, nanostructuring must be preserved under operational conditions (e.g., in quantum dot LEDs, lasers, photodetectors, and nanogranular thermoelectric devices). In other cases, we use nanocrystalline particles as precursors to a material with large crystalline grains and aim to sinter them as efficiently as possible (e.g., in polycrystalline thin-film solar cells). We carried out a systematic study of sintering and grain growth in materials composed of various sub-10 nm semiconductor grains. The boundaries between individual semiconductor grains have been chemically engineered using inorganic surface ligands. We found that the early stages of sintering and grain growth of nanocrystalline semiconductors are controlled by the ion mobility at the nanocrystal surfaces, while the late stages of grain growth are controlled by the mobility of the grain boundaries. This appears to be a general phenomenon for semiconductor nanocrystals, and it leads to several interesting and counterintuitive trends. For example, III–V InAs nanocrystals are generally much more resilient against sintering and grain growth compared to II–VI CdSe nanocrystals even though bulk CdSe has significantly higher melting point temperature than InAs (1268 °C vs 942 °C). Grain growth can be dramatically accelerated when coupled to solid−solid phase transitions. These findings expand our toolbox for rational design of nanocrystal materials for different applications

Erastin induces mPTP opening in cultured colorectal cancer cells.

<p>HT-29 cells were treated with applied erastin for indicated time, mPTP opening was evidenced by mitochondrial VDAC-1-ANT-1 association (A), cytochrome C (“Cyto-C”) release (B) and JC-10 intensity increase (C). HT-29 cells were pre-treated with sanglifehrin A (SfA, 2.5 μM), cyclosporin A (CsA, 0.5 μM) or bongkrekic acid (BA, 5 μM) prior to erastin (10 μM) treatment, cell survival (D) and apoptosis (E) were analyzed afterwards. Stably HT-29 cells expressing VDAC-1 shRNA-1/-2 or scramble control shRNA (“scr shRNA”) were treated with erastin (10 μM), VDAC-1 expression (F), cell survival (G) and apoptosis (H) were tested. ANT-1-assocaited Cyp-D (A), cytosol cytochrome C expression (B) and VDAC-1 expression (F) were quantified. For each assay, n = 4. The data presented were mean ± SD. Experiments were repeated three times with similar results obtained. * p < 0.05 vs. group of “Ctrl”. ^# p < 0.05 vs. group of erastin only (D and E) or the “scr shRNA” group (G and H). “Trans” stands for transfection control (F-H).</p

VDAC-1 over-expression potentiates erastin’s cytotoxicity.

<p>Stably HT-29 cells or NCM460 colon epithelial cells expressing empty vector (pSuper-puro, “Vec”) or VDAC-1 cDNA (“VDAC-1”) were treated with designed erastin for applied time, VDAC-1 expression, cell survival and apoptosis were tested by Western blotting assay (A and F), MTT assay (B and G) and histone DNA ELISA assay (C), respectively; ROS production (D) and JC-10 intensity (E) were also analyzed. For each assay, n = 4. The data presented were mean ± SD. Experiments were repeated three times with similar results obtained. VDAC-1 expression was quantified (F).* p < 0.05 vs. Ctrl group of “Vec” cells (B-E). ^# p < 0.05 vs. erastin group of “Vec” cells (B-E). “Trans” stands for transfection control (D and E).</p

Erastin induces ROS production and caspase-dependent apoptosis in cultured colorectal cancer cells.

<p>Colorectal cancer cells (HT-29, DLD-1 and Caco-2 lines) or NCM460 colon epithelial cells were treated with vehicle control (0.1% DMSO, “Ctrl”) or indicated concentrations of erastin for applied time, cell apoptosis was examined by listed assays (A-C, G and H); ROS production was also examined (D and I). HT-29 cells were pre-treated with z-DEVD-fmk (“zDEVD”, 50 μM), z-LEHD-fmk (“zLEHD”, 50 μM) or MnTBAP (10 μM) for 1 hour prior to applied erastin stimulation, cell survival and cell death were tested by MTT assay (E) and trypan blue assay (F), respectively. For each assay, n = 5. The data presented were mean ± SD. Experiments were repeated three times with similar results obtained. * p < 0.05 vs. group of “Ctrl”. ^# p < 0.05 vs. group of erastin only (E and F).</p

Solution-Processed Transistors Using Colloidal Nanocrystals with Composition-Matched Molecular “Solders”: Approaching Single Crystal Mobility

Crystalline silicon-based complementary metal-oxide–semiconductor transistors have become a dominant platform for today’s electronics. For such devices, expensive and complicated vacuum processes are used in the preparation of active layers. This increases cost and restricts the scope of applications. Here, we demonstrate high-performance solution-processed CdSe nanocrystal (NC) field-effect transistors (FETs) that exhibit very high carrier mobilities (over 400 cm²/(V s)). This is comparable to the carrier mobilities of crystalline silicon-based transistors. Furthermore, our NC FETs exhibit high operational stability and MHz switching speeds. These NC FETs are prepared by spin coating colloidal solutions of CdSe NCs capped with molecular solders [Cd₂Se₃]^2– onto various oxide gate dielectrics followed by thermal annealing. We show that the nature of gate dielectrics plays an important role in soldered CdSe NC FETs. The capacitance of dielectrics and the NC electronic structure near gate dielectric affect the distribution of localized traps and trap filling, determining carrier mobility and operational stability of the NC FETs. We expand the application of the NC soldering process to core–shell NCs consisting of a III–V InAs core and a CdSe shell with composition-matched [Cd₂Se₃]^2– molecular solders. Soldering CdSe shells forms nanoheterostructured material that combines high electron mobility and near-IR photoresponse

Erastin exerts cytotoxic, but not cytostatic, effects to cultured colorectal cancer cells.

<p>Colorectal cancer cells (HT-29, DLD-1 and Caco-2 lines) or NCM460 colon epithelial cells were treated with vehicle control (0.1% DMSO, “Ctrl”) or indicated concentrations of erastin for applied time, cell survival was tested by MTT assay (A and E) and colony formation assay (C); The percentage of trypan blue positive (“dead” cells) was recorded (B); Cell proliferation was tested by BrdU incorporation assay (D and F). For each assay, n = 5. The data presented were mean ± SD. Experiments were repeated three times with similar results obtained. * p < 0.05 vs. group of “Ctrl”.</p