CdSe Nanocrystals Research Articles

Explaining the Unusual Photoluminescence of Semiconductor Nanocrystals Doped via Cation Exchange.

Aliovalent doping of CdSe nanocrystals (NCs) via cation exchange processes has resulted in interesting and novel observations for the optical and electronic properties of the NCs. However, despite over a decade of study, these observations have largely gone unexplained, partially due to an inability to precisely characterize the physical properties of the doped NCs. Here, electrostatic force microscopy was used to determine the static charge on individual, cation-doped CdSe NCs in order to investigate their net charge as a function of added cations. While the NC charge was relatively insensitive to the relative amount of doped cation per NC, there was a remarkable and unexpected correlation between the average NC charge and PL intensity, for all dopant cations introduced. We conclude that the changes in PL intensity, as tracked also by changes in NC charge, are likely a consequence of changes in the NC radiative rate caused by symmetry breaking of the electronic states of the nominally spherical NC due to the Coulombic potential introduced by ionized cations.

Partitioning surface ligands on nanocrystals for maximal solubility

A typical colloidal nanoparticle can be viewed as a nanocrystal-ligands complex with an inorganic single-crystalline core, the nanocrystal, bonded with a monolayer of organic ligands. The surface chemistry of nanocrystal-ligands complexes is crucial to their bulk properties. However, deciphering the molecular pictures of the nonperiodic and dynamic organic-inorganic interlayer is a grand technical challenge, and this hampers the quantitative perception of their macroscopic phenomena. Here we show that the atomic arrangement on nanocrystal surface and ligand-ligand interactions can be precisely quantified through comprehensive solid-state nuclear magnetic resonance (SSNMR) methodologies. The analyses reveal that the mixed ligands of n-alkanoates on a CdSe nanocrystal segregate in areal partitions and the unique arrangement unlocks their rotational freedom. The mathematical model based on the NMR-derived ligand partition and dynamics successfully predicts the unusual solubility of nanocrystal-ligands complexes with mixed ligands, which is several orders of magnitude higher than that of nanocrystal-ligands complexes with pure ligands.

Long-Term Stability of Optical Properties of Colloidal CdSe Nanocrystals in Polymer Matrices

Properties of CdSe nanocrystals (NCs) fabricated in aqueous solutions at relatively mild conditions in the presence of gelatine and stabilized in polymer (gelatine, gelatine + polyvinylpyrrolidone, polyvinyl alcohol) matrices are characterized by Raman, optical absorption and photoluminescence spectroscopies. The effect of heating of the reaction mixture on the average NC size is discussed.

Role of Chalcogens in the Exciton Relaxation Dynamics of Chalcogenol-Functionalized CdSe QD: A Time-Domain Atomistic Simulation

Colloidal CdSe nanocrystals are often stabilized by organic ligands. The choice of such ligands has tremendous detrimental effects on interparticle charge transfer (CT) dynamics in nanocrystalline thin-film devices. It is evident from the recent experiment that photoexcited hole migrates from CdSe quantum dot (QD) to surface-passivating phenyl chalcogenol ligands (PhEH; E = S, Se, Te) at different time scales. But the backward electron–hole (e−h) recombination at the interface remained unexplored. A deep-level understanding of the mechanism of CT at the interface is therefore required to unravel the key role of chalcogen for the betterment of the device performance. Herein, we have performed time-domain density functional theory calculation along with nonadiabatic molecular dynamics (NAMD) simulation to investigate the photoinduced CT at the CdSe–PhEH interfaces. The simulated time scales for hole transfer (HT) are found to follow the trend PhSH > PhSeH > PhTeH that concur excellently with the experimenta...

Spontaneous Formation of CdSe Photoluminescent Nanotubes with Visible-Light Photocatalytic Performance.

Two-dimensional (2D) colloidal CdSe nanocrystals (NCs) with precise atomic-scale thickness have attracted intensive attention in recent years due to their optical properties and quantum confinement effects originating from their particular band structure. Here, we report a solution-based and template-free protocol to synthesize CdSe nanotubes (NTs) having 3–6 walls, each of which has 3.5 molecular monolayers. Their crystal structure is zincblende, with Cd-terminated {100} planes at the top and bottom surfaces of each wall, which are passivated by short-chain acetate ligands. After verifying the prominent role of the acetate ligand for NT synthesis, we elucidated the formation mechanism of these NTs. It starts by heterogeneous nucleation of 2D plateletlike nanoseeds from the amorphous Cd precursor matrix, followed by the growth via lateral and angular attachment of nanoplatelet building blocks into curved nanosheets, eventually resulting in NTs with sharp absorption and photoluminescence peak at around 460 nm. Moreover, the NTs show remarkable visible-light photocatalytic activity, as demonstrated by the reduction of the reddish Rhodamine B into its leuco form with a conversion rate of 92% in 1 min.

(Invited) Plasmonic Nanocrystal Composites for Photocatalytic Production of Solar Fuels

The visible extinction of noble metal nanoparticles surpasses those of semiconductor and molecular dyes by several orders of magnitude. Such superior light-harvesting characteristics are highly attractive for photocatalytic applications. Of a particular interest is the process of the plasmon near-field energy conversion, which is predicted to yield substantial gains in the photocarrier generation. Here, we employ the sample-transmitted excitation photoluminescence (STEP) spectroscopy to determine the quantum efficiency for the plasmon induced resonant energy transfer (ET) in assemblies of Au nanoparticles and semiconductor quantum dots (CdSe, PbS, etc.). The present technique distinguishes the Au-to-QD ET contribution from metal-induced quenching processes thus enabling accurate estimates of the photon-to-exciton conversion efficiency. We show that in the case of 9.1-nm Au nanoparticles, only 1-2% of the Au absorbed radiation is converted to excitons in the surrounding CdSe nanocrystal matrix. For larger, 21.0-nm Au, the photon-to-exciton conversion efficiency increases to 29.5%. The results of present measurements were used to develop an empirical model for estimating the maximum gain in the plasmon-induced carrier generation versus the mass-fraction of Au.

Correlated spatially resolved two-dimensional electronic and linear absorption spectroscopy.

A multimodal method for correlating linear and nonlinear optical spectra with a spatial resolution is presented. Using a partially collinear pump-probe geometry and two-frame phase-cycling, ultrafast two-dimensional electronic spectroscopy (2DES) was performed with transverse-spatial and temporal resolutions of 17μm and 80fs, respectively. Time-resolved 2DES maps were spatially correlated with linear extinction spectra obtained in the same imaging platform, enabling the examination of state-resolved dynamics of spatially heterogeneous materials. Thin films of aggregated CdSe nanocrystals were studied to demonstrate the combined spectral, temporal, and imaging capabilities of this method.

Double-exponential current–voltage (I–V) and negative capacitance (NC) behavior of Al/(CdSe-PVA)/p-Si/Al (MPS) structure

In this study, I–V and C–V characteristics of the Al/(CdSe-PVA)/p-Si/Al (MPS) structure have been investigated the by taking into account double-exponential I–V and NC behavior. The structural characterization of CdSe nanocrystals were analyzed by X-ray diffraction (XRD), scanning electron micros-copy (SEM), and energy dispersive X-ray (EDX) techniques. These results show that the prepared CdSe consists of spherical monodispersed nanocrystalites of about 200 nm; aggregated in the form of poly-dispersive nanoclusters of arbitrary shape with the size in the range of 150-300 nm and they are in good agreement with those estimated from the XRD pattern. The forward bias LnI–V plot of the MPS structure has two linear parts which are called low bias Region 1 (R1:0.25-0.65 V) and moderate bias Region 2 (R2:0.70-1.20 V). The ideality factor (n) and barrier height (BH:ΦB0) were found to be 9.38 and 0.61 eV for R1 and 6.51 and 0.65 eV for R2, respectively. The current conduction/transport mechanism (CCM/CTM) are also defined by forward I–VLn(I)–V and reverse bias In(IR)–VR1/2 plots. I–VLn(I)–V plot has also two linear parts which are also called (R1:-3.02/-0.29) and (R2:-0.10/1.60) obey I ~ Vm law. The slope (m) of them was found to be 1.63 and 4.57 which are corresponding to ohmic and trap-charge limited current (TCLC) mechanisms, respectively. Moreover, the value of ΦB(C–V) was found from the linear part of C−2-V plot as 0.79 eV which is higher than the obtained forward bias lnI–V plot due to the nature of measured method.

Effect of Bond Dispersion on Raman Spectra Shift in II-VI Semiconductor Nanocrystals.

To understand Raman spectra shifts of nanocrystals, the top-down phonon confinement approach and the bottom-up quantum chemical approach were developed. The former is suitable for large-sized nanocrystals, and the latter is suitable for clusters containing fewer atoms. Here, we find that a simpler chemical bond model based on the bond dispersion feature can demonstrate Raman spectra shift either in normal size II-VI semiconductor nanocrystals or in atomically precise clusters. According to the bond dispersion model, the Raman spectral line of the II-VI semiconductor nanocrystal (AIIBVI) is expressed as the sum of the Lorentz subpeaks of the AII( i)BVI( j) bonds with different coordinates i and j. The calculated Raman lines of CdSe, CdS, CdTe, ZnS, and ZnSe nanocrystals are in agreement with the measured Raman spectral lines. The origin of the red shift and asymmetric broadening of the peak position of nanocrystals may be revealed as well. Results provide insight into how different bonds contribute to different vibrational spectra.

Opto-electronics properties of starch capped CdSe nanoparticles

The nanoparticles of a prominent luminescent material, cadmium selenide (CdSe) have been prepared by simple wet chemical route using starch as capping agent. The X-ray diffraction of the synthesized CdSe nanocrystals exhibits hexagonal phase with average crystallite size decreasing with capping agent concentration. Surface morphology of CdSe nanoparticles has been studied using scanning electron microscopy. The UV–Vis absorption shows blue shift in absorption edge indicating increased band gap with decreasing the particle size due to quantum confinement. The photoluminescence and electroluminescence (EL) measurement show single peak at about 650 nm with increasing intensity for smaller nanocrystals, however emission peak is quite broad in case of EL indicating involvement of impurity states. The luminescence intensity increases because of increased oscillator strength and the better passivation of the surface defects responsible for non-radiative relaxation process. The light emission starts at lower threshold voltage when the concentration of capping agent is increased.

A novel strategy for boosting the photoluminescence quantum efficiency of CdSe nanocrystals at room temperature

Highly luminescent colloidal nanocrystals have wide applications in bioimaging and various optoelectronic devices. Herein we report a facile and mild procedure by combining S2− treatment and binary ligand passivation, which can efficiently enhance the luminescent property of CdSe nanocrystals at room temperature. The photoluminescence quantum yield of as-treated CdSe nanocrystals exhibits drastic enhancement (e.g., 188 times for CdSe nanorods) after this dual-passivation treatment. The methodology proposed here can be applied to various CdSe nanocrystals, regardless of their sizes, shapes, and crystal structures.

Thermal Excitation Control over Photon Emission Rate of CdSe Nanocrystals.

Temperature-dependent photoluminescence lifetimes of electron-hole pairs (excitons) in CdSe nanocrystals are governed by the energetic ordering and spacing of slowly emitting, spin-forbidden "dark" exciton states and rapidly emitting "bright" states. Here, infrared pulses that are resonant with hydrocarbon surface ligand vibrational transitions are shown to offer a route to manipulate the instantaneous emission rate of CdSe nanocrystals at cryogenic temperature. Transient heating of the inorganic nanocrystal core is achieved via resonant excitation of ligand vibrations, followed by heat flow to the nanocrystal lattice. Heating of the nanocrystal core is demonstrated using transient absorption spectroscopy, which shows a time-dependent red-shift of the quantum dot electronic absorption resonances, consistent with heating. Transient heating of the nanocrystal above the bath temperature increases the instantaneous radiative rate of the nanocrystals via a combination of thermal occupation of bright states as well as phonon-assisted emission. The lifetime of this infrared-pumped, fast-emitting sample condition is dictated by particle thermalization, which is multiple orders of magnitude shorter lived than the dark exciton state. This work demonstrates the feasibility of using heat control pulses to manipulate electronic recombination rates of excitons.

Controlling defect state emission in ultra-small sized tellurium doped CdSe nanocrystals via two-phase synthesis method

The utilization of semiconductor nanocrystals as white light emission source draws intense attention since semiconductor nanocrystals are promising candidates to fabricate new generation light emitting diodes. However, to synthesize high quality white light emitting semiconductor nanocrystals under mild reaction conditions at single step is a challenge. Here, we synthesized tellurium doped cadmium selenide (Te-doped CdSe) nanocrystals by using two phase synthesis method under mild reaction conditions at single step. Random localization of tellurium atoms in cadmium selenide nanocrystals caused emergence of defect state emission. The defect state emission was controlled by tuning initial Te:Se molar ratio in aqueous phase of the two phase system. Luminescence behaviour of Te-doped CdSe nanocrystals was affected by defect state emission and therefore, dual light emission (composition of blue-green light, whitish color) was observed. The Te-doped CdSe had cubic structure with Te composition at most 10% with size ranging around 1.8 nm. The CRI 1931 coordinates of the Te-doped CdSe nanocrystals revealed that Te-doped CdSe nanocrystals can be a powerful constituent for new generation light emitting diodes.

Photophysical Action Spectra of Emission from Semiconductor Nanocrystals Reveal Violations to the Vavilov Rule Behavior from Hot Carrier Effects

Semiconductor nanocrystals are known to have properties of bulk semiconductors as well as molecules. Two rules that govern molecules are that there is no dual emission (Kasha) and there is no spectrum to the emission quantum yield (Vavilov). We show that the latter rule of molecular spectroscopy is generally violated in semiconductor nanocrystals. Through experiments and theory on CdSe and perovskite nanocrystals, these violations are shown to arise via hot carrier effects. Experiments and simple phenomenology reveal that quantum yield spectra arise because of enhanced hot carrier trapping rates. A semiclassical electron-transfer theory rationalizes a microscopic picture of the carrier kinetics. These effects are especially significant when quantifying syntheses of bright emitters such as perovskite nanocrystals. These effects are also a general approach to simple steady-state measurements of the action of hot carrier kinetics.

Direct Observation of Vibronic Coupling between Excitonic States of CdSe Nanocrystals and Their Passivating Ligands

Here, we report resonance Raman spectra of CdSe colloidal nanocrystals (NCs) passivated with organic ligands. In addition to the well-known longitudinal optical phonons, we observe ligand vibrations. The ligand vibrations are shown to be resonantly enhanced through electronic mixing with the states of the NC. These measurements were enabled by substituting the native ligands with thiophenol. Thiophenol serves as an ideal probe for exciton–ligand coupling as it is a widely employed Raman molecular tag and quenches background luminescence in CdSe. The ligand vibrations are shown to be resonantly enhanced through exciting NC transitions. We show that vibronic coupling is observable in CdSe with diameters from 2 to 6 nm and for both phosphonic acid and amine native ligands. The coupling is evidenced by both asymmetric and symmetric mode enhancement through Herzberg–Teller or Franck–Condon and Herzberg–Teller mechanisms, repectively. The ligand exchange quenching strategy may be generally applicable to study ...

Air-Stable CuInSe2 Nanocrystal Transistors and Circuits via Post-Deposition Cation Exchange.

Colloidal semiconductor nanocrystals (NCs) are a promising materials class for solution-processable, next-generation electronic devices. However, most high-performance devices and circuits have been achieved using NCs containing toxic elements, which may limit their further device development. We fabricate high mobility CuInSe2 NC field-effect transistors (FETs) using a solution-based, post-deposition, sequential cation exchange process that starts with electronically coupled, thiocyanate (SCN)-capped CdSe NC thin films. First Cu+ is substituted for Cd2+ transforming CdSe NCs to Cu-rich Cu2Se NC films. Next, Cu2Se NC films are dipped into a Na2Se solution to Se-enrich the NCs, thus compensating the Cu-rich surface, promoting fusion of the Cu2Se NCs, and providing sites for subsequent In-dopants. The liquid-coordination-complex trioctylphosphine-indium chloride (TOP-InCl3) is used as a source of In3+ to partially exchange and n-dope CuInSe2 NC films. We demonstrate Al2O3-encapsulated, air-stable CuInSe2 NC FETs with linear (saturation) electron mobilities of 8.2 ± 1.8 cm2/(V s) (10.5 ± 2.4 cm2/(V s)) and with current modulation of 105, comparable to that for high-performance Cd-, Pb-, and As-based NC FETs. The CuInSe2 NC FETs are used as building blocks of integrated inverters to demonstrate their promise for low-cost, low-toxicity NC circuits.

Quantized Electronic Doping towards Atomically Controlled "Charge-Engineered" Semiconductor Nanocrystals.

"Charge engineering" of semiconductor nanocrystals (NCs) through so-called electronic impurity doping is a long-standingchallenge in colloidal chemistry and holds promise for ground-breaking advancements in many optoelectronic, photonic, and spin-based nanotechnologies. To date, our knowledge is limited to a few paradigmatic studies on a small number of model compounds and doping conditions, with important electronic dopants still unexplored in nanoscale systems. Equally importantly, fine-tuning of charge engineered NCs is hampered by the statistical limitations of traditional approaches. The resulting intrinsic doping inhomogeneity restricts fundamental studies to statistically averaged behaviors and complicates the realization of advanced device concepts based on their advantageous functionalities. Here we aim to address these issues by realizing the first example of II-VI NCs electronically doped with an exact number of heterovalent gold atoms, a known p-type acceptor impurity in bulk chalcogenides. Single-dopant accuracy across entire NC ensembles is obtained through a novel non-injection synthesis employing ligand-exchanged gold clusters as "quantized" dopant sources to seed the nucleation of CdSe NCs in organic media. Structural, spectroscopic, and magneto-optical investigations trace a comprehensive picture of the physical processes resulting from the exact doping level of the NCs. Gold atoms, doped here for the first time into II-VI NCs, are found to incorporate as nonmagnetic Au+ species activating intense size-tunable intragap photoluminescence and artificially offsetting the hole occupancy of valence band states. Fundamentally, the transient conversion of Au+ to paramagnetic Au2+ (5d9 configuration) under optical excitation results in strong photoinduced magnetism and diluted magnetic semiconductor behavior revealing the contribution of individual paramagnetic impurities to the macroscopic magnetism of the NCs. Altogether, our results demonstrate a new chemical approach toward NCs with physical functionalities tailored to the single impurity level and offer a versatile platform for future investigations and device exploitation of individual and collective impurity processes in quantum confined structures.

Effects of Ag doping on the electronic and optical properties of CdSe quantum dots.

Cadmium selenide (CdSe) nanocrystals are important photoelectric materials. Doping heterovalent impurities such as silver (Ag) in CdSe nanocrystal quantum dots (QDs) can provide additional charge carriers, which can significantly enhance the performance of CdSe QDs for their potential applications in high-efficiency photovoltaic devices. Using density functional theory (DFT) based calculations with the Heyd-Scuseria-Ernzerhof (HSE06) screened hybrid functional, we demonstrate that Ag doping can affect the structural, electronic and optical properties of CdSe QDs significantly. The location and number of Ag dopant atoms are critical factors for modifying the electronic structure, in particular the change of energy position and shape of the valence and conduction band edges. It is found that doping of Ag atoms into the core region of a CdSe nanoparticle induces metallic-like electronic characteristics with a dense number of electrons emerging at the Fermi level. However, incorporation of Ag dopant into the surface of a CdSe quantum dot introduces some mid-gap states that mainly consist of Se 4p states, and results in a new sub-bandgap electronic transition from mid-gap states to the conduction band. The calculated absorption spectra indicate that doping of just one or two Ag atoms greatly strengthens the absorption in the ultraviolet-visible regime and extends the absorption edges of CdSe QDs into the infrared regime. In particular, the spectra show a high-intensity absorption band between 424 and 600 nm with just 1 Ag atom incorporated into the CdSe QDs. Based on the improved absorption spectra, the present results provide a science-based strategy for designing Ag-doped CdSe QDs with enhanced visible light absorption for their application in high-efficiency photovoltaic devices.

Direct cation exchange of CdSe nanocrystals into ZnSe enabled by controlled binding between guest cations and organic ligands.

Zn chalcogenides are suitable candidates for blue-emitting fluorophores in light-emitting devices. In particular, the efforts to grow ZnSe nanocrystals (NCs) with fine control over size and shape via bottom-up approaches have faced challenges because of the slow decomposition of Zn precursors. In this study, we report direct cation exchange from CdSe NCs to ZnSe. Absorption spectroscopy and density functional theory (DFT) analysis reveal that the reactivity of cation exchange depends on the degree of complexation between organic ligands and Zn halides. We controlled the binding strength of Zn complexes by changing the organic ligands and halogen species that bind with Zn. Appropriate binding strength allows for the release of Zn ions and their facile incorporation into CdSe seed NCs. Under our experimental conditions, trioctylphosphine oxide (TOPO)-ZnI2 drives the efficient cation exchange reaction whereas TOPO-ZnCl2 induces no cation exchange of CdSe NCs. In addition, functional groups vary the binding strength between Zn and ligands. Oleylamine (OLAm)-ZnI2, which has a weaker ligand-ZnI2 binding than TOPO-ZnI2, breaks down the original morphologies of host CdSe NCs due to the very fast exchange rate. On the other hand, the TOPO-ZnI2 complex induces a mild exchange rate, leading to transformation into various morphologies such as CdSe nanorods (NRs) and nanoplatelets (NPLs) into CdSe/ZnSe heterostructures inaccessible via other synthesis methods. The incorporation of Zn into various morphologies of CdSe results in tunable optical transitions in blue-UV regions. The synthesis of heterostructured NCs in an elongated morphology is possible, opening opportunities in photocatalysis, light emitting diodes, and luminescent solar concentrators.

Correction: Protic additives determine the pathway of CdSe nanocrystal growth.

Correction for 'Protic additives determine the pathway of CdSe nanocrystal growth' by Klaus Boldt et al., Nanoscale, 2018, 10, 18238-18248.