Jianying Ouyang

Low-temperature approach to highly emissive copper indium sulfide colloidal nanocrystals and their bioimaging applications.

We report our newly developed low-temperature synthesis of colloidal photoluminescent (PL) CuInS2 nanocrystals (NCs) and their in vitro and in vivo imaging applications. With diphenylphosphine sulphide (SDPP) as a S precursor made from elemental S and diphenylphosphine, this is a noninjection based approach in 1-dodecanethiol (DDT) with excellent synthetic reproducibility and large-scale capability. For a typical synthesis with copper iodide (CuI) as a Cu source and indium acetate (In(OAc)3) as an In source, the growth temperature was as low as 160 °C and the feed molar ratios were 1Cu-to-1In-to-4S. Amazingly, the resulting CuInS2 NCs in toluene exhibit quantum yield (QY) of ~23% with photoemission peaking at ~760 nm and full width at half maximum (FWHM) of ~140 nm. With a mean size of ~3.4 nm (measured from the vertices to the bases of the pyramids), they are pyramidal in shape with a crystal structure of tetragonal chalcopyrite. In situ (31)P NMR (monitored from 30 °C to 100 °C) and in situ absorption at 80 °C suggested that the Cu precursor should be less reactive toward SDPP than the In precursor. For our in vitro and in vivo imaging applications, CuInS2/ZnS core-shell QDs were synthesized; afterwards, dihydrolipoic acid (DHLA) or 11-mercaptoundecanoic acid (MUA) were used for ligand exchange and then bio-conjugation was performed. Two single-domain antibodies (sdAbs) were used. One was 2A3 for in vitro imaging of BxPC3 pancreatic cancer cells. The other was EG2 for in vivo imaging of a Glioblastoma U87MG brain tumour model. The bioimaging data illustrate that the CuInS2 NCs from our SDPP-based low-temperature noninjection approach are good quality.

CdS magic-sized nanocrystals exhibiting bright band gap photoemission via thermodynamically driven formation.

CdS magic-sized nanocrystals (MSNs) exhibiting both band gap absorption and emission at 378 nm with a narrow bandwidth of approximately 9 nm and quantum yield (QY) of approximately 10% (total QY approximately 28%, in hexane) were synthesized via a one-pot noninjection approach. This CdS MSN ensemble is termed as Family 378. It has been acknowledged that magic-sized quantum dots (MSQDs) are single-sized, and only homogeneous broadening contributes to their bandwidth. The synthetic approach developed is ready and highly reproducible. The formation of the CdS MSQDs was carried out at elevated temperatures (such as 90-140 degrees C) for a few hours in a reaction flask containing bis(trimethylsilyl)sulfide ((TMS)(2)S) and Cd(OAc)(OA) in situ made from cadmium acetate dihydrate (Cd(OAc)(2).2H(2)O) and oleic acid (OA) in 1-octadecene (ODE). Low OA/Cd and high Cd/S feed molar ratios favor this formation, whose mechanism is proposed to be thermodynamically driven. (13)C solid-state cross-polarization magic-angle spinning (CP/MAS) nuclear magnetic resonance (NMR) demonstrates that the capping ligands are firmly attached to the nanocrystal surface via carboxylate groups. With the cross-polarization from (1)H of the alkyl chains to surface (113)Cd, (113)Cd NMR is able to distinguish the surface Cd (471 ppm) bonding to both -COO(-) and S and the bulk Cd (792 ppm) bonding to S only. DOSY-NMR was used to determine the size of Family 378 ( approximately 1.9 nm). The present study provides strategies for the rational design of various MSNs.

Low-temperature approach to high-yield and reproducible syntheses of high-quality small-sized PbSe colloidal nanocrystals for photovoltaic applications.

Small-sized PbSe nanocrystals (NCs) were synthesized at low temperature such as 50-80 °C with high reaction yield (up to 100%), high quality, and high synthetic reproducibility, via a noninjection-based one-pot approach. These small-sized PbSe NCs with their first excitonic absorption in wavelength shorter than 1200 nm (corresponding to size < ∼3.7 nm) were developed for photovoltaic applications requiring a large quantity of materials. These colloidal PbSe NCs, also called quantum dots, are high-quality, in terms of narrow size distribution with a typical standard deviation of ∼7-9%, excellent optical properties with high quantum yield of ∼50-90% and small full width at half-maximum of ∼130-150 nm of their band-gap photoemission peaks, and high storage stability. Our synthetic design aimed at promotion of the formation of PbSe monomers for fast and sizable nucleation with the presence of a large number of nuclei at low temperature. For formation of the PbSe monomer, our low-temperature approach suggests the existence of two pathways of Pb-Se (route a) and Pb-P (route b) complexes. Either pathway may dominate, depending on the method used and its experimental conditions. Experimentally, a reducing/nucleation agent, diphenylphosphine, was added to enhance route b. The present study addresses two challenging issues in the NC community, the monomer formation mechanism and the reproducible syntheses of small-sized NCs with high yield and high quality and large-scale capability, bringing insight to the fundamental understanding of optimization of the NC yield and quality via control of the precursor complex reactivity and thus nucleation/growth. Such advances in colloidal science should, in turn, promote the development of next-generation low-cost and high-efficiency solar cells. Schottky-type solar cells using our PbSe NCs as the active material have achieved the highest power conversion efficiency of 2.82%, in comparison with the same type of solar cells using other PbSe NCs, under Air Mass 1.5 global (AM 1.5G) irradiation of 100 mW/cm(2).

In-situ observation of nucleation and growth of PbSe magic-sized nanoclusters and regular nanocrystals.

In-situ observation of the temporal evolution of the absorption of PbSe nanocrystals (NCs) via a low-temperature noninjection approach is presented. Based on a model reaction of lead oleate (Pb(OA)(2) ) and n-trioctylphosphine selenide (TOPSe) in 1-octadecene at 35-80 °C, the use of commercially available TOP (90 or 97%) in affecting the formation of the NCs is explored. TOPSe solutions made from TOP 90% exhibited higher reactivity than those made from TOP 97%. (31)P NMR spectroscopy detected no dioctylphosphine selenide (DOPSe) but some DOP in ≈1.0 M TOPSe/TOP solution (made from TOP 90%), as well as no diphenylphosphine selenide (DPPSe) when DPP was added to the ≈1.0 M solution. Hence, it is proposed that, for the formation of PbSe monomers, an indirect pathway dominates with the formation of a Pb-P complex/intermediate, which results from the activation of Pb(OA)(2) by a phosphine compound (such as DPP, DOP, or TOP) and in turn reacts with TOPSe. With the use of TOP 90% and the addition of secondary phosphine DPP, the formation of PbSe magic-sized nanoclusters (MSNCs) and regular NCs (RNCs) is investigated. With proper tuning of the synthesis conditions, the formation of various PbSe MSNCs versus RNCs is monitored in situ with versus without the addition of DPP, or at different reaction temperatures but otherwise identical synthetic formulation and reaction parameters. Accordingly, the degree of supersaturation (DS) of the PbSe monomer affecting the development of these PbSe MSNCs versus RNCs is proposed; the higher the DS, the more the MSNCs are favored. Also, surface-determined cluster-cluster aggregation is proposed to be the growth mechanism for both the RNCs and MSNCs. For the former, quantized growth is followed by continuous growth. For the latter, the sizes of the magic-sized families are calculated.

Low-temperature noninjection approach to homogeneously-alloyed PbSe(x)S(1-x) colloidal nanocrystals for photovoltaic applications.

Homogeneously alloyed PbSe(x)S(1-x) nanocrystals (NCs) with their excitonic absorption peaks in wavelength shorter than 1200 nm were developed for photovoltaic (PV) applications. Schottky-type solar cells fabricated with our PbSe₀.₃S₀.₇ NCs as their active materials reached a high power conversion efficiency (PCE) of 3.44%, with an open circuit voltage (V(oc)) of 0.49 V, short circuit photocurrent (J(sc)) of 13.09 mA/cm², and fill factor (FF) of 0.54 under Air Mass 1.5 global (AM 1.5G) irradiation of 100 mW/cm². The syntheses of the small-sized colloidal PbSe(x)S(1-x) NCs were carried out at low temperature (60 °C) with long growth periods (such as 45 min) via a one-pot noninjection-based approach in 1-octadecene (ODE), featuring high reaction yield, high product quality, and high synthetic reproducibility. This low-temperature approach employed Pb(oleate)₂ as a Pb precursor and air-stable low-cost thioacetamide (TAA) as a S source instead of air-sensitive high-cost bis(trimethylsilyl)sulfide ((TMS)₂S), with n-tributylphosphine selenide (TBPSe) as a Se precursor instead of n-trioctylphosphine selenide (TOPSe). The reactivity difference of TOPSe made from commercial TOP 90% and TBPSe made from commercial TBP 97% and TBP 99% was addressed with in situ observation of the temporal evolution of NC absorption and with ³¹P nuclear magnetic resonance (NMR). Furthermore, the addition of a strong reducing/nucleation agent diphenylphosphine (DPP) promoted the reactivity of the Pb precursor through the formation of a Pb-P complex, which is much more reactive than Pb(oleate)₂. Thus, the reactivity of TBPSe was increased more than that of TAA. The larger the DPP-to-Pb feed molar ratio, the more the Pb-P complex, the higher the Se amount in the resulting homogeneously alloyed PbSe(x)S(1-x) NCs. Therefore, the use of DPP allowed reactivity match of the Se and S precursors and led to sizable nucleation at low temperature so that long growth periods became feasible. The present study brings insight into the formation mechanism of monomers, nucleation/growth of colloidal composition-tunable NCs, and materials design and synthesis for next-generation low-cost and high-efficiency solar cells.

The Formation Mechanism of Binary Semiconductor Nanomaterials: Shared by Single‐Source and Dual‐Source Precursor Approaches

One thing in common: The formation of binary colloidal semiconductor nanocrystals from single- (M(EEPPh2 )n ) and dual-source precursors (metal carboxylates M(OOCR)n and phosphine chalcogenides such as E=PHPh2 ) is found to proceed through a common mechanism. For CdSe as a model system (31) P NMR spectroscopy and DFT calculations support a reaction mechanism which includes numerous metathesis equilibriums and Se exchange reactions.

Highly-photoluminescent ZnSe nanocrystals via a non-injection-based approach with precursor reactivity elevated by a secondary phosphine.

Highly-photoluminescent ZnSe quantum dots with 72% quantum yield and 22 nm full width at half maximum were synthesized with more reactive precursors via a non-injection approach with high synthetic reproducibility; (31)P NMR provided insight into the formation mechanisms of ZnSe monomers.

Ultraviolet ZnSe1–xSx Gradient-Alloyed Nanocrystals via a Noninjection Approach

Highly emissive ultraviolet ZnSeS nanocrystals (NCs), with a core-shell-like structure, were designed and synthesized via a one-step noninjection approach in 1-octadecene (ODE). These ultraviolet ZnSeS NCs exhibit bright bandgap emission with high color purity and little trap emission. With full width at half-maximum (fwhm) of ∼21 nm only, photoluminescent (PL) quantum yield (QY) of ∼60% was estimated for one ensemble dispersed in toluene exhibiting bandgap absorption peaking at ∼380 nm and bandgap emission at ∼389 nm. These alloyed ZnSeS NCs present a cubic crystal structure consisting of a Se-rich core and a S-rich shell. Such a gradiently alloyed structure was suggested by our investigation on the temporal evolution of optical properties of the growing ZnSeS NCs monitored from 80 to 300 °C, together with structural and compositional characterization performed with XRD, XPS, EDX, and TEM. This newly developed one-step noninjection approach was achieved with zinc oleate (Zn(OA)(2)), diphenylphosphine selenide (SeDPP), and diphenylphosphine sulfide (SDPP) as Zn, Se, and S precursors, respectively. ZnSe monomers mainly participated in nucleation at ∼120 °C, while both ZnSe and ZnS monomers contributed to NC formation in later growth stages (∼160 °C and higher). (31)P NMR study demonstrates that SeDPP is more reactive than SDPP toward Zn(OA)(2), and also supports such a model proposed on the combination of ZnSe and ZnS monomers leading to nucleation/growth of ZnSeS alloyed NCs. The present study offers conceptual methodology to various highly photoluminescent alloyed NCs with high quality, high particle yield, and high synthetic reproducibility.