High Quality CdTe Research Articles

Synthesis of Cadmium Telluride Quantum Wires and the Similarity of Their Effective Band Gaps to Those of Equidiameter Cadmium Telluride Quantum Dots

High-quality colloidal CdTe quantum wires having purposefully controlled diameters in the range 5-11 nm are grown by the solution-liquid-solid (SLS) method, using Bi nanoparticle catalysts, cadmium octadecylphosphonate and trioctylphosphine telluride as precursors, and a TOPO solvent. The wires adopt the wurtzite structure and grow along the [002] direction (parallel to the c axis). The size dependence of the effective band gaps in the wires is determined from the absorption spectra and compared to the experimental results for high-quality CdTe quantum dots. In contrast to the predictions of an effective-mass approximation, particle-in-a-box model, and previous experimental results from CdSe and InP dot-wire comparisons, the effective band gaps of CdTe dots and wires of like diameter are found to be experimentally indistinguishable. The present results are analyzed using density functional theory under the local-density approximation by implementing a charge-patching method. The higher-level theoretical analysis finds the general existence of a threshold diameter, above which dot and wire effective band gaps converge. The origin and magnitude of this threshold diameter are discussed.

Synthesis and Bio-Imaging Application of Highly Luminescent Mercaptosuccinic Acid-Coated CdTe Nanocrystals

Here we present a facile one-pot method to prepare high-quality CdTe nanocrystals in aqueous phase. In contrast to the use of oxygen-sensitive NaHTe or H2Te as Te source in the current synthetic methods, we employ more stable sodium tellurite as the Te source for preparing highly luminescent CdTe nanocrystals in aqueous solution. By selecting mercaptosuccinic acid (MSA) as capping agent and providing the borate-citrate acid buffering solution, CdTe nanocrystals with high quantum yield (QY >70% at pH range 5.0–8.0) can be conveniently prepared by this method. The influence of parameters such as the pH value of the precursor solution and the molar ratio of Cd2+ to Na2TeO3 on the QY of CdTe nanocrystals was systematically investigated in our experiments. Under optimal conditions, the QY of CdTe nanocrystals is even high up to 83%. The biological application of luminescent MSA-CdTe to HEK 293 cell imaging was also illustrated.

A facile and green preparation of high-quality CdTe semiconductor nanocrystals at roomtemperature

One chemical reagent, hydrazine hydrate, was discovered to accelerate the growth ofsemiconductor nanocrystals (cadmium telluride) instead of additional energy, which wasapplied to the synthesis of high-quality CdTe nanocrystals at room temperature andambient conditions within several hours. Under this mild condition the mercapto stabilizerswere not destroyed, and they guaranteed CdTe nanocrystal particle sizes with narrowand uniform distribution over the largest possible range. The CdTe nanocrystals(photoluminescence emission range of 530–660 nm) synthesized in this way hadvery good spectral properties; for instance, they showed high photoluminescencequantum yield of up to 60%. Furthermore, we have succeeded in detecting the livingBorrelia burgdorferi of Lyme disease by its photoluminescence image using CdTenanocrystals.

A novel synthesis of high quality CdTe quantum dots with good thermal stability

A facile method for the synthesis of the highly luminescent CdTe quantum dots (QDs) is presented in this paper. Paraffin liquid and oleic acid were used as solvent and ligand instead of trioctylphosphine oxide (TOPO) and phosphonic acids (HPA or TDPA) to synthesize various sized (3.2–6 nm) CdTe QDs under mild conditions and the photoluminescence quantum yield (PLQY) of the as-prepared CdTe QDs can reach as high as 65%. Moreover, the thermal stability of different sized CdTe QDs were also investigated, and the results show the as-prepared CdTe QDs with larger size are more sensitive to the temperature than those with smaller size.

Novel Strategy for Synthesis of High Quality CdTe Nanocrystals in Aqueous Solution

A novel aqueous route for the synthesis of high-quality CdTe nanocrystals(NCs) is presented in this article. With both glutathione(GSH) and cysteine[n(GSH):n(cysteine)=1:3] as stabilizers, high-quality CdTe NCs with controllable photoluminescence(PL) wavelength from 500 to 630 nm can be prepared within 4 h. As-prepared CdTe NCs show higher photoluminescence quantum yields(PLQY) compared with CdTe NCs prepared via other aqueous methods. When the fluorescent emission peak appeared in orange-red window, the PLQY reaches 70% or above at room temperature without any post-preparative treatment.

Hydrothermal synthesis for high-quality CdTe quantum dots capped by cysteamine

We present a facile hydrothermal approach to synthesize high-quality cysteamine (CA)-capped CdTe quantum dots (QDs). Oil bath heating and vigorous stirring were used to obtain better heat transfer and more homogenous solutions during the synthesis process. By this approach, the quantum yield (QY) of the resultant QDs can reach as high as 19.7%, which is the best reported data for CA-stabilized CdTe QDs. The synthesis process is under a high concentration of the precursor (> 10 mM), suggesting the potential of this route to be used in mass production of CA-capped CdTe QDs. Moreover, the pH-dependent optical properties of CA-capped CdTe QDs were also investigated.

Ablation of CdTe with 100 µs Nd:YAG laser pulses: dependence on target preparation method

AbstractThe results of experimental studies of the ablation of CdTe with a pulsed Nd:YAG laser (wavelength 1064 nm) performed with 100 µs pulses and repetition time of 35 Hz are presented for the pulse energy range from 0.13 to 0.25 J. The main goal is to elucidate the dependence of the ablation process on the target preparation method. The investigation of the vapour stream intensity and chemical composition and their evolution with time are performed with a quadrupole mass spectrometer synchronized with the laser pulses. These studies are performed for three kinds of targets: a target made of CdTe bulk crystal (BC target), a target made of CdTe fine powder pressed under the pressure of 700 atm (PP target), and a target made of loose CdTe powder (N‐PP target). The applicability of these targets for obtaining high quality CdTe thin films is determined. The best chemical composition of the vapour stream can be obtained with the BC target. A major drawback of this target is the energetic threshold for ablation with Nd:YAG laser and resulting delay in the ablation process above the threshold. The advantage of powder targets over BC target is the lack of any ablation threshold or delay. Weaker angular dependence of the particle emission (associated with the surface roughness), if confirmed in further experiments, can be the most important advantage of PP and N‐PP targets. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Low temperature growth of high quality CdTe polycrystalline layers

We have investigated the growth of CdTe thin films on glass substrates by hot wall epitaxy. The layers have been characterized by scanning electron microscopy, atomic force microscopy, profilometry, x-ray diffraction and optical transmission. The grown samples are polycrystalline with a high preferential [1 1 1] orientation. Atomic force microscopy and scanning electron microscopy reveal pyramidal grain shapes with a size of around 0.3 µm. The surface roughness increases with sample thickness and growth temperature, reaching about 200 nm for 10 µm thick layers grown at 300 °C. Samples with a thickness of 2 µm grown at 150 °C showed a roughness of less than 40 nm. Optical transmission measurements demonstrate layers with high optical quality.

Selective Growth of CdTe on Si(211): First-Principle Calculations

The adsorption of CdTe layers on clean and As-passivated Si(211) substrates has been simulated by first-principle calculations in this study. Based on the simulation results, we theoretically show the important roles of the As4 passivation during the epitaxial growth. Arsenic can saturate part of the dangling bonds and weaken the surface states. The partial passivation finally induces the B-face polarity selection automatically. This conclusion can provide further explanations for the successful growth of large area high-quality CdTe(211)B layers on the Si(211) substrates.

A new Approach for the Preparation of High Quality CdTe Nanocrystals and their Optical Characterization

A new synthesis procedure for the preparation of CdTe nanocrystals is presented, exhibiting bright luminescence with exceptionally high quantum efficiency (85%). The growth of these nanocrystals occurs in a non-coordinating mother solution, octadecen, with the addition of oleic acid/tri- octylphosphine stabilizers, CdO as a monomer and additional Cd metal particles as an additional Cd reservoir source. The produced NCs have a spherical shape with 5% size distribution and cubic zinc blende crystallographic structure. The absorption and photoluminescence excitation spectra exhibit four distinct excitonic bands at the spectral regime between 530-630 nm. The non-resonantly excited PL spectra show an excitonic band with a full width at half maximum of ~30 nm, Stokes shifted by 75 meV to 36 meV from the lowest exciton transition, decreasing with an increase in the average diameter of the nanocrystals. The fluorescence line narrowing spectra are comprised of a zero-phonon band and a longitudinal-optical replica.

Microwave-Assisted Synthesis of Water-Dispersed CdTe Nanocrystals with High Luminescent Efficiency and Narrow Size Distribution

Water-dispersed nanocrystals with high photoluminescence quantum yield (PLQY) and narrow size distribution are desirable for a variety of bioapplications. In this report, we present a facile method assisted by microwave irradiation for rapidly synthesizing high-quality CdTe nanocrystals in aqueous phase. The PLQY of CdTe nanocrystals prepared in very short time (15 min) reaches as high as 82%, and the fwhm value is merely 27 nm meanwhile. Moreover, the PLQY is increased to a remarkable 98% after further amelioration through the illumination method. The key synthesis parameters (molar ratio of reagents, pH values, reaction temperature, and time) are fully discussed, and the optimum conditions are illuminated in this paper. In addition, the mechanism about microwave irradiation improving the quality of nanocrystals is discussed as well.

The selective synthesis of water-soluble highly luminescent CdTe nanoparticles and nanorods: The influence of the precursor Cd/Te molar ratio

In this report, we initially systematically investigated the influence of the precursor Cd/Te molar ratio on the morphology of water-soluble thiol-stabilized CdTe nanocrystals. By only changing the precursor Cd/Te molar ratio, highly luminescent CdTe nanoparticles (NPs) and nanorods (NRs) can be prepared selectively in the presence of the same concentration single ligand ( l-cysteine or thioglycolic acid) system. A high precursor Cd/Te molar ratio leads to isotropic spherical growth, whereas a low Cd/Te molar ratio promotes the linear self-assembly of NPs into NRs. Thus, a new efficient strategy has been developed in aqueous phase to prepare selectively highly luminescent dot- or rod-shaped CdTe nanocrystals in single ligand system. Then, we further explored the influence of the properties of initial CdTe dispersions with different luminescence maxima on the formation of NRs. The experiment results revealed that the formation of CdTe NRs is also dependent on the properties of initial CdTe dispersions. CdTe NPs with short wavelength emission (∼520–550 nm) can self-assemble directly into high quality CdTe NRs after storage at room temperature.

Factors Governing the Quality of Aqueous CdTe Nanocrystals: Calculations and Experiment

The aqueous synthesis of thiol-stabilized semiconductor CdTe colloidal nanocrystals has been revisited. We found optimal conditions for the synthesis of high-quality CdTe NCs through a study of the influence of the initial conditions (structure and concentration of Cd-thiol complexes) on the quality of the CdTe nanocrystals. A numerical calculation shows a clear correlation between the concentration of CdL (where L is (SCH(2)COO)(2-)) in the initial solution and the photoluminescence quantum efficiency of the CdTe nanocrystals.

Comparison of Water-Soluble CdTe Nanoparticles Synthesized in Air and in Nitrogen

It is commonly believed that high-quality CdTe nanoparticles with strong luminescence can only be prepared under the protection of an inert gas such as nitrogen or argon. Here, we report the preparation of highly luminescent CdTe nanoparticles in air and compare their luminescence properties with CdTe nanoparticles made in nitrogen. We find that both water-soluble CdTe nanoparticles made in air and in nitrogen exhibit strong photoluminescence as well as upconversion luminescence at room temperature. However, differences do exist between the particles made in air and those made in nitrogen. In particular, the particles prepared in air display a faster growth rate, grow to larger sizes, and display stronger electron coupling relative to the particles prepared in nitrogen. X-ray photoelectron spectroscopy analysis indicates that the oxygen content in the nanoparticles synthesized in air is higher that that in particles synthesized in N(2), likely resulting in a higher availability of excess free cadmium. Cytotoxicity measurements reveal that the particles made in air appear slightly more toxic, possibly due to the excess of free cadmium.

Interface Interaction Controlled Transport of CdTe Nanoparticles in the Microcontact Printing Process

High-quality CdTe nanoparticles stabilized with thioglycolic acid (TGA) are patterned on SiO2/Si surfaces using microcontact printing (microCP). Due to the weak interaction of the nanoparticles with the stamp surface, tailoring of gas flow rate during the inking process as well as the type and scale of the patterns on the stamp are used to control the distribution of the nanoparticles on the structured stamp surface. This distribution is then transferred the printed regions. Either edge printing or homogeneous printing can be achieved under optimized conditions. In addition, new structures such as nanowires form under certain conditions.

Synthesis of CdTe Nanocrystals through Program Process of Microwave Irradiation

A novel method, the program process of microwave irradiation (PPMI), which commendably integrates good qualities of microwave irradiation and a programmed heating process, is presented to synthesize high-quality CdTe nanocrystals in aqueous solution. Microwave irradiation, which acts as the heating mode of PPMI, is highly favorable for a narrow size distribution and low concentration of surface defects of nanocrystals. On the other hand, two correlative processes (the first process and the second process) are utilized in PPMI to actualize the programmed heating process, which is an effective strategy to improve the quality of nanocrystals. Thus, a series (diameters approximately 2-4 nm) of highly luminescent (PLQY approximately 30-68%) CdTe nanocrystals were rapidly prepared (reaction time approximately 1-30 min) in aqueous phase through PPMI.

Direct growth of high-quality thick CdTe epilayers on Si (211) substrates by metalorganic vapor phase epitaxy for nuclear radiation detection and imaging

Direct growth of high-quality, thick CdTe (211) epilayers, with thickness up to 100 µm, on Si (211) substrates in a vertical metalorganic vapor phase epitaxy system is reported. In order to obtain homo-orientation growth on Si substrates, pretreatment of the substrates was carried out in a separate chamber by annealing them together with pieces of GaAs at 800–900°C in a hydrogen environment. Grown epilayers had very good substrate adhesion. The full-width at half-maximum (FWHM) value of the x-ray double-crystal rocking curve from the CdTe (422) reflection decreased rapidly with increasing layer thickness and remained between 140–200 arcsec for layers >18 µm. Photoluminescence measurement at 4.2 K showed high-intensity, bound excitonic emission and very small defect-related deep emissions, indicating the high crystalline quality of the grown layers. Furthermore, a CdTe/n+-Si heterojunction diode was fabricated that exhibited clear rectifying behavior.

The structure of the Si (211) surface

Silicon (211) has been proposed as an alternative substrate for CdTe/HgCdTe molecular beam epitaxial growth. Silicon has a clear advantage over other substrates because of its low cost, high strength, and thermal-expansion coefficient, which matches that of the silicon readout integrated circuit. The (211) orientation has been shown to yield high-quality CdTe and HgCdTe/CdTe layers over other orientations. The reconstruction and faceting of the Si (211) surface is poorly understood despite the importance of the (211) orientation. The results of low-energy electron diffraction (LEED) studies have been contradictory, and their conclusions are inconsistent with recent scanning tunneling microscopy (STM) studies. LEED and STM images were used to determine the most probable Si (211) surface facet structure as a function of annealing temperature. Samples annealed at a high temperature (i.e., >1260°C) allowed the formation of ordered LEED spot patterns as opposed to the typically reported \([\bar 111]\) streaks. The pattern in the \([0\bar 11]\) direction gave a consistent 2× (7.68 A) reconstruction.

Growth of CdTe∕Si(100) thin films by pulsed laser deposition for photonic applications

Cadmium telluride (CdTe) has become one of the most useful and versatile photonic materials. While many applications require the highest quality material possible, others can sacrifice some degree of sample quality in favor of reduced fabrication costs. Here, we explore the deposition of CdTe thin films deposited on (100) silicon substrates in the absence of costly ultrahigh vacuum conditions. This inevitably leads to the deposition of films on silicon’s native oxide. The work presented here explores the trade-offs in sample quality as determined by a host of characterization techniques. The films were deposited using the pulsed laser deposition technique at relatively low growth rates and at an optimized substrate temperature of 300°C. X-ray diffraction data show only (111) oriented CdTe with rocking curve widths broadened due to the lack of an epitaxial relationship between the film and substrate. Atomic force microscopy images confirm that the films have a smooth surface morphology as was suggested by their mirrorlike appearance. Film quality was also analyzed using photoluminescence, positron annihilation spectroscopy, and ellipsometry. While structural deficiencies have been observed in these films, the optical properties are remarkably similar to those expected for high quality CdTe.

Characterization of charge collection in CdTe and CZT using the transient current technique

The charge collection properties of Cadmium–Telluride (CdTe) and Cadmium–Zinc–Telluride (CZT) in comparison with Silicon (Si) are presented using the transient-current technique (TCT) where the current pulses are generated by α -particles emitted from an 241Am source. From the recorded current pulse shapes, the charge collection efficiency, the charge carrier mobility and the electric field distribution inside the detectors are extracted. In particular, the signals of the compound semiconductors CdTe and CZT are interpreted with respect to the build-up of space–charges in the sensor volume and the subsequent deformation of the electric field. As high-quality CdTe and CZT samples are now commercially available, the knowledge of these material characteristics is of outmost importance for the application of CdTe and CZT in X-ray imaging. In addition, the paper describes the influence of Ohmic and Schottky contacts on the current pulses in CdTe as well as the effects of polarization, i.e. the time-dependent degradation of the detector signals due to the accumulation of fixed charges within the sensor.