A Facile Phosphine-Free Method for Synthesizing PbSe Nanocrystals with Strong Optical Limiting Effects

Wang H., Pei Y., LaLonde A.D., Snyder G.J.

PbSe is a surprisingly good thermoelectric material due, in part, to its low thermal conductivity that had been overestimated in earlier measurements. The thermoelectric figure of merit, zT , can exceed 1 at high temperatures in both p-type and n-type PbSe, similar to that found in PbTe. While the p-type lead chalcogenides (PbSe and PbTe) benefit from the high valley degeneracy (12 or more at high temperature) of the valence band, the n-type versions are limited to a valley degeneracy of 4 in the conduction band. Yet the n-type lead chalcogenides achieve a zT nearly as high as the p-type lead chalcogenides. This effect can be attributed to the weaker electron–phonon coupling (lower deformation potential coefficient) in the conduction band as compared with that in the valence band, which leads to higher mobility of electrons compared to that of holes. This study of PbSe illustrates the importance of the deformation potential coefficient of the charge-carrying band as one of several key parameters to consider for band structure engineering and the search for high performance thermoelectric materials.

Sun L., Choi J.J., Stachnik D., Bartnik A.C., Hyun B., Malliaras G.G., Hanrath T., Wise F.W.

Infrared light-emitting diodes are currently fabricated from direct-gap semiconductors using epitaxy, which makes them expensive and difficult to integrate with other materials. Light-emitting diodes based on colloidal semiconductor quantum dots, on the other hand, can be solution-processed at low cost, and can be directly integrated with silicon. However, so far, exciton dissociation and recombination have not been well controlled in these devices, and this has limited their performance. Here, by tuning the distance between adjacent PbS quantum dots, we fabricate thin-film quantum-dot light-emitting diodes that operate at infrared wavelengths with radiances (6.4 W sr(-1) m(-2)) eight times higher and external quantum efficiencies (2.0%) two times higher than the highest values previously reported. The distance between adjacent dots is tuned over a range of 1.3 nm by varying the lengths of the linker molecules from three to eight CH(2) groups, which allows us to achieve the optimum balance between charge injection and radiative exciton recombination. The electroluminescent powers of the best devices are comparable to those produced by commercial InGaAsP light-emitting diodes. By varying the size of the quantum dots, we can tune the emission wavelengths between 800 and 1,850 nm.

Liu Y., Yao D., Shen L., Zhang H., Zhang X., Yang B.

Enhancement of Se solubility in organic solvents without the use of alkylphosphine ligands is the key for phosphine-free synthesis of selenide semiconductor nanocrystals (NCs). In this communication, we demonstrate the dissolution of elemental Se in oleylamine by alkylthiol reduction at room temperature, which generates soluble alkylammonium selenide. This Se precursor is highly reactive for hot-injection synthesis of selenide semiconductor NCs, such as Cu2ZnSnSe4, Cu(InGa)Se2, and CdSe. In the case of Cu2ZnSnSe4, for example, the as-synthesized NCs possessed small size, high size monodispersity, strong absorbance in the visible region, and in particular a promising increase in photocurrent under AM1.5 illumination. The current preparation of the Se precursor is simple and convenient, which will promote the synthesis and practical applications of selenide NCs.

Manga K.K., Wang J., Lin M., Zhang J., Nesladek M., Nalla V., Ji W., Loh K.P.

Highly sensitive, multicomponent broadband photodetector devices are made from PbSe/graphene/TiO(2). TiO(2) and PbSe nanoparticles act as light harvesting photoactive materials from the UV to IR regions of the electromagnetic spectrum, while the graphene acts as a charge collector for both photogenerated holes and electrons under an applied electric field.

Fu H., Tsang S.

Simple solution phase, catalyst-free synthetic approaches that offer monodispersed, well passivated, and non-aggregated colloidal semiconductor nanocrystals have presented many research opportunities not only for fundamental science but also for technological applications. The ability to tune the electrical and optical properties of semiconductor nanocrystals by manipulating the size and shape of the crystals during the colloidal synthesis provides potential benefits to a variety of applications including photovoltaic devices, light-emitting diodes, field effect transistors, biological imaging/labeling, and more. Recent advances in the synthesis and characterization of colloidal lead chalcogenide nanocrystals and the achievements in colloidal PbS or PbSe nanocrystals solar cells have demonstrated the promising application of infrared-emitting colloidal lead chalcogenide nanocrystals in photovoltaic devices. Here, we review recent progress in the synthesis and optical properties of colloidal lead chalcogenide nanocrystals. We focus in particular upon the size- and shape-controlled synthesis of PbS, PbSe, and PbTe nanocrystals by using different precursors and various stabilizing surfactants for the growth of the colloidal nanocrystals. We also summarize recent advancements in the field of colloidal nanocrystals solar cells based on colloidal PbS and PbSe nanocrystals.

Kim D.K., Lai Y., Vemulkar T.R., Kagan C.R.

We report low-hysteresis, ambipolar bottom gold contact, colloidal PbSe nanowire (NW) field-effect transistors (FETs) by chemically modifying the silicon dioxide (SiO(2)) gate dielectric surface to overcome carrier trapping at the NW-gate dielectric interface. While water bound to silanol groups at the SiO(2) surface are believed to give rise to hysteresis in FETs of a wide range of nanoscale materials, we show that dehydration and silanization are insufficient in reducing PbSe NW FET hysteresis. Encapsulating PbSe NW FETs in cured poly(methyl) methacrylate (PMMA), dehydrates and uniquely passivates the SiO(2) surface, to form low-hysteresis FETs. Annealing predominantly p-type ambipolar PbSe NW FETs switches the FET behavior to predominantly n-type ambipolar, both with and without PMMA passivation. Heating the PbSe NW devices desorbs surface bound oxygen, even present in the atmosphere of an inert glovebox. Upon cooling, overtime oxygen readsorption switches the FET polarity to predominantly p-type ambipolar behavior, but PMMA encapsulation maintains low hysteresis. Unfortunately PMMA is sensitive to most solvents and heat treatments and therefore its application for nanostructured material deposition and doping is limited. Seeking a robust, general platform for low-hysteresis FETs we explored a variety of hydroxyl-free substrate surfaces, including silicon nitride, polyimide, and parylene, which show reduced electron trapping, but still large hysteresis. We identified a robust dielectric stack by assembling octadecylphosphonic acid (ODPA) on aluminum oxide (Al(2)O(3)) to form low-hysteresis FETs. We further integrated the ODPA/Al(2)O(3) gate dielectric stack on flexible substrates to demonstrate low-hysteresis, low-voltage FETs, and the promise of these nanostructured materials in flexible, electronic circuitry.

Jawaid A.M., Asunskis D.J., Snee P.T.

Developing simple synthetic methods to control the size and morphology of nanocrystals is an active area of research as these parameters control the material’s electronic and optical properties. For a semiconductor with a symmetrical crystal structure such as lead selenide, anisotropic colloidal growth has been previously accomplished via the use of templates, seeds, or by block assembly of smaller, symmetrical subunits. Here, we present a simple method to create monodisperse lead selenide nanorods and multipods at low temperatures. The size distribution and the observed morphologies are consistent with a continuous, anisotropic growth of material. The syntheses of these anisotropic shapes are due to the nature of the nuclei that form upon injection of precursors into partially oxidized alkene solvents that may contain lactone and carbonate-functional derivatives.

Androulakis J., Todorov I., He J., Chung D., Dravid V., Kanatzidis M.

We report promising thermoelectric properties of the rock salt PbSe-PbS system which consists of chemical elements with high natural abundance. Doping with PbCl(2), excess Pb, and Bi gives n-type behavior without significantly perturbing the cation sublattice. Thus, despite the great extent of dissolution of PbS in PbSe, the transport properties in this system, such as carrier mobilities and power factors, are remarkably similar to those of pristine n-type PbSe in fractions as high as 16%. The unexpected finding is the presence of precipitates ~2-5 nm in size, revealed by transmission electron microscopy, that increase in density with increasing PbS concentration, in contrast to previous reports of the occurrence of a complete solid solution in this system. We report a marked impact of the observed nanostructuring on the lattice thermal conductivity, as highlighted by contrasting the experimental values (~1.3 W/mK) to those predicted by Klemens-Drabble theory at room temperature (~1.6 W/mK). Our thermal conductivity results show that, unlike in PbTe, optical phonon excitations in PbSe-PbS systems contribute to heat transport at all temperatures. We show that figures of merit reaching as high as ~1.2-1.3 at 900 K can be obtained, suggesting that large-scale applications with good conversion efficiencies are possible from systems based on abundant, inexpensive chemical elements.

Hyun B., Bartnik A.C., Koh W., Agladze N.I., Wrubel J.P., Sievers A.J., Murray C.B., Wise F.W.

Measurements of the far-infrared absorption spectra of PbSe nanocrystals and nanorods are presented. As the aspect ratio of the nanorods increases, the Fröhlich sphere resonance splits into two peaks. We analyze this splitting with a classical electrostatic model, which is based on the dielectric function of bulk PbSe but without any free-carrier contribution. Good agreement between the measured and calculated spectra indicates that resonances in the local field factors underlie the measured spectra.

Bolotin I.L., Asunskis D.J., Jawaid A.M., Liu Y., Snee P.T., Hanley L.

Oleic acid capped lead sulfide (PbS) and lead selenide (PbSe) were synthesized, then subjected to a postsynthesis washing in a 1:1 ethanol/hexane solution. The relationship of third order nonlinear optical properties to nanocrystal surface chemistry as affected by washing was analyzed using nanosecond 532 nm Z-scan and measurement of near-IR radiative emission. The results indicated a significant change in optical nonlinearities which emerged only after the nanocrystals were washed in the ethanol/hexane mixture. Transmission electron microscopy of the oleic acid capped PbSe and PbS nanocrystals showed a cubic shape, narrow size distribution and an average size of ∼10 nm. Neither size nor shape of the nanocrystals were modified by the washing process, indicating that all optical differences were related to changes in surface chemistry and the formation of deep trap states. The “as grown” nanocrystals showed high emission efficiency, weak saturable absorption, and a self-defocusing refractive index, n2, det...

Sun M., Yang X.

CdSe nanocrystals (NCs) are prepared in noncoordination solvents (1-octadecene (ODE) and paraffin liquid) with long-chain primary alkylamine as the sole ligand, ODE-Se, and cadmium fatty acid salt as precursors. The obtained NCs meet the four fundamental parameters for high-quality NCs: high crystallinity, narrow size distribution, moderate photoluminescence quantum yield, and broad range size tunableness. Further, by simply regulating the relative molar ratio of alkylamine to cadmium precursor, the regular sized “nuclei” and final obtained NCs can be produced predictably within a certain size range. The size distribution of regular sized “nuclei” is very narrow (fwhm = 23 ± 1 nm), and the following focusing growth procedure vanishes. This indicates a different nucleation and growth kinetics from that of the well-established “focusing” and “defocusing” theory. By analyzing the conversion factor of precursors and the concentration of magic sized nanoclusters and regular sized “nuclei”, a subtle secondary n...

Pietryga J.M., Werder D.J., Williams D.J., Casson J.L., Schaller R.D., Klimov V.I., Hollingsworth J.A.

Infrared-emitting nanocrystal quantum dots (NQDs) have enormous potential as an enabling technology for applications ranging from tunable infrared lasers to biological labels. Notably, lead chalcogenide NQDs, especially PbSe NQDs, provide efficient emission over a large spectral range in the infrared, but their application has been limited by instability in emission quantum yield and peak position on exposure to ambient conditions. Conventional methods for improving NQD stability by applying a shell of a more stable, wider band gap semiconductor material are frustrated by the tendency of lead chalcogenide NQDs toward Ostwald ripening at even moderate reaction temperatures. Here, we describe a partial cation-exchange method in which we take advantage of this lability to controllably synthesize PbSe/CdSe core/shell NQDs. Critically, these NQDs are stable against fading and spectral shifting. Further, these NQDs can undergo additional shell growth to produce PbSe/CdSe/ZnS core/shell/shell NQDs that represent initial steps toward bright, biocompatible near-infrared optical labels.

Jasieniak J., Bullen C., van Embden J., Mulvaney P.

High quality CdSe nanocrystals have been prepared using elemental selenium as the chalcogenide precursor dispersed in 1-octadecene (ODE). The conditions used to prepare the Se precursor were found to be critical for successful nanocrystal synthesis. Systematic titration of the Se precursor solution with tri-n-octylphosphine (TOP) allowed the Se reactivity to be tuned and the final particle size to be controlled. Band-edge and surface related emission were observed for samples prepared in the presence and absence of added TOP. In the absence of a selenium passivant, the crystal structure of CdSe nanocrystals could be altered from zinc blende to wurtzite by the addition of bis(2,2,4-trimethylpentyl)phosphinic acid (TMPPA).

Lu W., Fang J., Ding Y., Wang Z.L.

In this paper we report an electron microscopic observation of crystal shape development when PbSe nanocrystals were synthesized using a dynamic injection technique at different temperatures in the presence of oleic acid. A two-step evolution mechanism was proposed, indicating that the shape evolution of PbSe nanocrystals is dependent on the growth time, whereas the crystalline size can be tuned by varying the growth temperature under the studied conditions. It also implies that a higher growth rate in the 111 direction compared to that in the 100 direction results in the formation of nanocubes.

Cho K., Talapin D.V., Gaschler W., Murray C.B.

Single-crystal PbSe nanowires are synthesized in solution through oriented attachment of nanocrystal building blocks. Reaction temperatures of 190−250 °C and multicomponent surfactant mixtures result in a nearly defect-free crystal lattice and high uniformity of nanowire diameter along the entire length. The wires' dimensions are tuned by tailoring reaction conditions in a range from ∼4 to ∼20 nm in diameter with wire lengths up to ∼30 μm. PbSe nanocrystals bind to each other on either {100}, {110}, or {111} faces, depending on the surfactant molecules present in the reaction solution. While PbSe nanocrystals have the centrosymmetric rocksalt lattice, they can lack central symmetry due to a noncentrosymmetric arrangement of Pb- and Se-terminated {111} facets and possess dipole driving one-dimensional oriented attachment of nanocrystals to form nanowires. In addition to straight nanowires, zigzag, helical, branched, and tapered nanowires as well as single-crystal nanorings can be controllably prepared in one-pot reactions by careful adjustment of the reaction conditions.

Huang Y., Wang J., Bai B., Zhao M., Zhen X., Zhao L., Zhai X., Zhao L., Leng X.

Yang C., Zhang X., Li P., Chen S., Feng S.

Lead selenide (PbSe) has been extensively studied as an infrared detection material capable pf operating at room temperature. In this research, we proposed a simple cyclic voltammetry (CV) electrodeposition of hierarchical porous PbSe on single-layer graphene film. Scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy were used to evaluate the physical and surface characteristics of porous PbSe films. The porous structure with large surface to volume ratio will enhance the light absorption ability after effectively sensitized in oxygen atmosphere. Furthermore, oxygen-sensitized processes create electron traps at grain boundaries, extending the lifetime of the hole carrier and improving photoconductive sensitivity. At room temperature, the porous PbSe/graphene photodetector exhibit a responsivity of 12.3 A/W, a detectivity of 5.95 * 108 cm Hz1/2W−1 (@1 kHz) at 1.55 µm, and a responsivity of 15.2 A/W, a detectivity of 1.4 * 109 cm Hz1/2W−1(@1 kHz) at 2.7 µm. This work highlights the low cost and effective electrodeposition strategy for constructing hierarchical porous structure with further application in infrared photodetection.

Ponomarenko Vladimir P., Popov Victor, Shuklov Ivan, Ivanov Victor V., Razumov Vladimir F.

Photosensing based on colloidal quantum dots (CQDs) is a rapidly developing area of infrared photoelectronics. The use of colloidal quantum dots markedly simplifies the manufacture, decreases the restrictions to the pixel pitch of the photosensitive elements, and reduces the production cost, which facilitates the wide use of IR sensors in various technological systems. This paper is the first exhaustive overeview of the architectures, methods of manufacturing and basic properties of photonic sensors based on colloidal quantum dots of compounds of Group II, IV and VI elements. Characteristic features of the synthesis and roles of the ligands and CQD morphology in the design of photosensors are considered in detail. The structures of photoresistive, photodiode and phototransistor elements based on HgTe, HgSe, PbS and PbSe CQDs, which are sensitive in various spectral ranges, are described. The main parameters of the most advanced optoelectronic devices based on colloidal quantum dot structures are presented. The key trends in the development of this area are analyzed.The bibliography includes 361 references.

Neisius N.A., MacHale L.T., Snyder E.R., Finke R.G., Prieto A.L.

Fu J., Yang C., Nie C., Sun F., Li G., Feng S., Wei X.

AbstractLead sulfide (PbS), a classical narrow‐bandgap material, has manifested itself as a reliable choice for short‐wave infrared (SWIR) detection with high sensitivity. The primary efforts of optimizing PbS‐based photodetectors rely on discovering new low‐dimensional morphology and designing delicate device architecture, which may raise problems in the synergistic improvement of response speed, sensitivity, and response region. Herein, a graphene‐PbS nanocuboids‐graphene vertical photodetector is reported, fabricated by in situ electrochemical growth of monocrystalline PbS nanocuboids on graphene using an electrochemical technique. Under illumination, the surface states of PbS nanocuboids trap photogenerated holes and form a photovoltage, which widens the conductive region in the channel. This photogating effect contributes to the photocurrent and renders the device with high responsivity. Moreover, the highly crystalline PbS nanocuboids serve as excellent vertical channels, allowing a fast speed to be achieved. At room temperature, the device shows a responsivity of ≈130 A W−1 at 2700 nm, an external quantum efficiency of ≈6000%, a detectivity of 3 × 109 cm Hz1/2 W−1 (@1 kHz), and response/recovery speeds of 15/42 ms. This vertically integrated device, composed of PbS nanocuboids and graphene, offers a promising approach to complementary metal oxide semiconductor (CMOS)‐compatible SWIR detection, thus opening up an avenue to tailor device performance by exploiting the anisotropic morphology of nanomaterials.

Zhai X., Ma B., Wang Q., Zhang H.

Two-dimensional materials are now excelling in yet another arena of ultrafast photonics, including optical modulation through optical limiting/mode-locking, photodetectors, optical communications, integrated miniaturized all-optical devices, etc.

Shuklov I.A., Razumov V.F.

The review concerns the state of the art in methods of synthesis of colloidal lead chalcogenide quantum dots (QDs). The most recent data on the mechanisms of chemical transformations involving various precursors are discussed. Particular attention is paid to the influence of (i) trace impurities in the reactants used and (ii) post-synthesis treatment on the physicochemical properties of QDs used in photoelectric devices. The bibliography includes 129 references.

Li N., Wang Q., Zhang H.

The development of two-dimensional (2D) materials have attracted increasing interest due to their unique structure and various potential applications such as opto-electronic devices and photocatalysis. Our group have contributed to this exciting field by creating novel preparation methods for a various of 2D materials including transition metal dichalcogenides (TMDs), carbon nitrides and single elemental 2D materials from Group 15. Particularly, employing powerful time-resolved spectroscopic techniques such as femtosecond transient absorption spectroscopy, we elucidated the excited-state dynamics of 2D materials behind their outstanding performance in photocatalytic and photonic devices. Therefore in this account, we focus on the effective fabrication methods of 2D materials and their photoinduced excited-state dynamics. Following the introduction in Part 1, we will summarize our novel strategies for fabricating 2D materials (Part 2). Then in Part 3 we will introduce the instrumentation for exploring the photoinduced excited-state dynamics of the 2D materials spanning a wide time scale from ultrafast fs to slow ms. Part 4 details the applications of the 2D materials in photocatalysis and nonlinear optics determined by their excited-state physics and chemistry. Part 5 of perspectives summarizes a few future trends of 2D materials on a series of issues like fabrications, dynamic investigations and photonic optoelectronic applications. Collective efforts through researchers from interdisciplinary fields are expected to further push the exciting territory towards a new horizon.

Hou X., Sun J., Liu Z., Yan C., Song W., Zhang H., Zhou S., Shao X.

Ring reconstruction of bowl-shaped trichalcogenasumanenes results in diimide compounds which show excellent optical limiting performance superior to C60.

Zhao Y., Tian S., Wang Z., Zhu M., Ren H., Ma Q., Guo Z., Li K., Ding S., Miao Z.

Nowadays, with the increasing use of high-power laser, research and development of laser-damage prevention materials with multifunctional features and wideband wavelengths are urgent and indispensa...

Li Y., Du H., Zhang J., Liu Z., Tian M., Che R.

The physical properties of nanometer scale semiconductors are known to be sensitively influenced by their aspect ratios, but the intrinsic mechanisms still remain unclear. Shape-controlled anisotropic PbSe nanorods were obtained by means of the addition of MnCl2, and the aspect ratio of the nanorods can be continuously tuned from 1 to 10 by simply modulating the amount of chloride ions. It was demonstrated that an optimized concentration of Cl- anions is about 0.04mmol, which controls the competition between thermodynamics and kinetics mechanisms. The emission peaks of the infrared absorbance and photoluminescence spectra were significantly tuned from 1664nm to 1840nm and from 1459nm to 1938nm only by the aspect ratios, respectively. A strong electric dipole phenomenon localized onside the surface of PbSe nanorods terminated by Pb2+ charge was found by using high-spatial-resolution off-axis electron holography, which was furthermore evidenced by the quantitative analysis of the mean inner potential and the surfaces charge. The charge intensity depended on the aspect ratio of PbSe nanorods. The results provide clear evidence that the energy gap interval reduces as a result of the increasing of conduction charge amounts. A novel strategy to facilely shift the peak position of absorbance and photoluminescence emission was therefore proposed.

Dini D., Calvete M.J., Hanack M.

The control of luminous radiation has extremely important implications for modern and future technologies as well as in medicine. In this Review, we detail chemical structures and their relevant photophysical features for various groups of materials, including organic dyes such as metalloporphyrins and metallophthalocyanines (and derivatives), other common organic materials, mixed metal complexes and clusters, fullerenes, dendrimeric nanocomposites, polymeric materials (organic and/or inorganic), inorganic semiconductors, and other nanoscopic materials, utilized or potentially useful for the realization of devices able to filter in a smart way an external radiation. The concept of smart is referred to the characteristic of those materials that are capable to filter the radiation in a dynamic way without the need of an ancillary system for the activation of the required transmission change. In particular, this Review gives emphasis to the nonlinear optical properties of photoactive materials for the function of optical power limiting. All known mechanisms of optical limiting have been analyzed and discussed for the different types of materials.

Zhao M., Liu K., Zhang Y., Wang Q., Li Z., Song Y., Zhang H.

Herein we discovered new tricks of pentacene derivatives, which exhibited strong optical limiting properties with high stability, a new benchmark significantly superior to the traditional fullerene.

Zhao M., Peng R., Zheng Q., Wang Q., Chang M., Liu Y., Song Y., Zhang H.

Graphene-based materials have shown promising nonlinear optical properties in the visible range. To extend their nonlinear optical response to the near infrared (NIR) region, we prepared a new nanohybrid consisting of uniform PbS quantum dots (QDs) attached on the reduced graphene oxide, named rGO-PbS, via a facile, low-cost, and phosphine-free method. The rGO-PbS nanohybrid exhibited superior optical limiting properties to either graphene oxide or PbS QDs upon both 532 nm and 1064 nm excitation in the nanosecond laser pulse regime, which is attributed to the synergetic effects stemming from charge transfer between the two components. Meanwhile, the thin films containing the rGO-PbS nanohybrid dispersed in polymethylmethacrylate (PMMA) also showed excellent optical limiting properties with high transparency, implying the potential applications of this hybrid material in broadband nonlinear optical devices.

Zhang Z., Liu C., Zhao X.

The lead chalcogenide quantum dots with a size smaller than 3 nm and good chemical stability were difficult to synthesize due to the poor chemical reactivity of commonly used phosphine-free chalcogen and lead sources. Here we report the synthesis of small and surface passivated PbSe quantum dots (QDs) by addition of SnCl2 as the nucleation promoting agent and PbCl2 as the in situ passivation agent. Common PbCl2 (dissolved in oleylamine, oleic acid, and 1-octadecene) and elemental Se (dissolved in oleylamine) were used to synthesize small PbSe QDs by addition of SnCl2 to promote the fast nucleation of PbSe nanocrystals. The sizes of the PbSe QDs are between 3 and 8 nm and are tunable by controlling the ratio of PbCl2/SnCl2 and the reaction time. High photostability of PbSe QDs approved by the XPS analysis and photoluminescence spectra of PbSe QDs was also achieved by the in situ formation of thin halide adlayers on the surface of QDs. This new method is green with accessible raw materials and simple in sit...