Al-doped ZnO Nanocrystals Research Articles

Dynamically modulating the mid-infrared localized surface plasmon resonance of Al-doped ZnO nanocrystals

The localized surface plasmon resonance (LSPR) regulation of semiconductor oxide nanocrystals (NCs) in the near-infrared region (0.8–2.5 μm) of the solar spectrum has attracted much attention for its potential in applications such as electrochromism and sensing. However, the LSPR modulation properties of doped semiconductor oxide NCs in the mid-infrared region (2.5–25 μm) have rarely been reported. Here, we synthesized aluminum-doped zinc oxide (AZO) NCs with various nominal Al doping contents (1–20at%). The LSPR absorption peaks of these films were regulated between ∼3.8 and 6.57 μm depending on the doping content. AZO NC films with 1at% nominal Al doping and a 1316 nm film thickness exhibited the highest electrochemical infrared absorption regulation, achieving 35.9% regulation at 3–5 μm and 32.5% at 7.5–13 μm. The infrared absorption regulation mechanism of the AZO NCs involved changes in the LSPR absorption intensity caused by electron injection/extraction into/from the surface depletion layer of the AZO NCs. After applying different voltages, the AZO NC film on the Al substrates exhibited different apparent temperatures, demonstrating its potential applications in infrared displays and camouflage.

Significant Enhancement in Electrical Conductivity of Al doped ZnO Nanocrystals

Significant Enhancement in Electrical Conductivity of Al doped ZnO Nanocrystals

Morphology, structure and emission of Al-doped ZnO nanocrystal films

The morphology, structure and emission of Al-doped ZnO nanocrystals (NCs) with the different Al contents (1–4 at.%) were studied by means of the scanning electronic microscopy, energy dispersive X ray spectroscopy, X-ray diffraction and photoluminescence (PL) methods. Ultrasonic spray pyrolysis was applied to obtain the ZnO:Al films. To stimulate the crystallization, the ZnO:Al films were annealed at 400 °C for 4 h in a constant nitrogen flow (8 L/min). It is shown that the Al incorporation in the ZnO films with the concentrations of 2–4 at.% stimulates: the reduction of ZnO:Al grain sizes, decreasing the film crystallinity owing to disordering the ZnO:Al crystal lattice, the change of the surface morphology and increasing the surface roughness. Meanwhile, Al-doping the ZnO films at the concentrations ≤ 2 at.% enlarge significantly the PL intensity of the near band edge emission. Last fact testifies to quality improving the ZnO:Al films. Simultaneously, the PL intensities of green and orange PL bands, connected with the native defects: VZn and Oi, fall down. The ZnO NC films with Al-doping ≤ 2 at.% still keep the planar surface morphology that is important for their applications in electronic device structures.

Estimation of accurate size, lattice strain using Williamson-Hall models, SSP and TEM of Al doped ZnO nanocrystals

Hexagonal structure of pure and Al doped ZnO nanocrystals were synthesized by co-precipitation technique. The average crystallite size determined from XRD data using Debye Sherrer’s formula was found to be 16.8 nm for pure 19.3 nm for 4% Al Doped ZnO. High-resolution transmission electron microscopy (HR-TEM) showed single-crystal ZnO nanocrystals with nearly spherical shapes. The W-H analysis and size-strain plot were used to study the individual contributions of crystallite sizes and lattice strain “ε” on the peak broadening of pure and Al doped ZnO nanocrystals. The physical parameters namely strain, stress and anisotropic energy density were calculated considering uniform deformation model (UDM), uniform stress deformation model (USDM) and uniform deformation energy density model (UDEDM) of modified form of W-H analysis. Texture coefficient was estimated using XRD data. The average crystallite size of pure and Al doped ZnO samples estimated from W-H model. It is also revealed from the TEM the diameters of as synthesized samples are in good agreement with the W-H model analysis.

Ligand‐Free Synthesis of Aluminum‐Doped Zinc Oxide Nanocrystals and their Use as Optical Spacers in Color‐Tuned Highly Efficient Organic Solar Cells

The color of polymer solar cells using an opaque electrode is given by the reflected light, which depends on the composition and thickness of each layer of the device. Metal‐oxide‐based optical spacers are intensively studied in polymer solar cells aiming to optimize the light absorption. However, the low conductivity of materials such as ZnO and TiO2 limits the thickness of such optical spacers to tenths of nanometers. A novel synthesis route of cluster‐free Al‐doped ZnO (AZO) nanocrystals (NCs) is presented for solution processing of highly conductive layers without the need of temperature annealing, including thick optical spacers on top of polymer blends. The processing of 80 nm thick optical spacers based on AZO nanocrystal solutions on top of 200 nm thick polymer blend layer is demonstrated leading to improved photocurrent density of 17% compared to solar cells using standard active layers of 90 nm in combination with thin ZnO‐based optical spacers. These AZO NCs also open new opportunities for the processing of high‐efficiency color tuned solar cells. For the first time, it is shown that applying solution‐processed thick optical spacer with polymer blends of different thicknesses can process solar cells of similar efficiency over 7% but of different colors.

A straightforward and “green” solvothermal synthesis of Al doped zinc oxide plasmonic nanocrystals and piezoresistive elastomer nanocomposite

Ethanol solvothermal method derives plasmonic Al doped ZnO nanocrystals suitable for advanced applications.

Aluminum doped ZnO sol–gel derived nanocrystals: Raman spectroscopy and solid solubility characterization

In this study, a sol–gel method was used to synthesize Al‐doped ZnO nanocrystals (AZO). For preparation of this highly concentrated sol, zinc acetate concentration was fixed at 2M. UV‐visible absorption and photoluminescence spectroscopy were used to evaluate optical properties of the prepared samples. High temperature annealing was employed to evaluate the solid solubility limit of Al ions. XRD and Raman spectroscopy are then used to evaluate the doping process of aluminum in ZnO nanopowders. Disappearing of the second order Raman peak of ZnO at 1160 cm−1 was used to confirm the solid solubility limit. It can be found that the disappearing of second order Raman peak intensity at 1160 cm−1 is observed for sample with 2 at% Al3+ ion. Experimental results have shown that the solid solubility limit of aluminium ions in sol–gel derived zinc oxide nanocrystals was achieved.

Low-temperature preparation of transparent conductive Al-doped ZnO thin films by a novel sol–gel method

We developed a novel sol–gel method to prepare transparent conductive Al-doped ZnO (AZO) thin film at low temperature. The AZO nanocrystals were prepared by a solvothermal method and then they were dispersed in the monoethanolamine and methanol to form AZO colloids. A (002)-oriented ZnO thin film was used as a nucleation layer to induce the (002)-oriented growth of AZO thin films. The AZO thin films were prepared on Si(100) and fused quartz glass substrates with the (002)-oriented ZnO nucleation layer and annealed at 400 °C for 60 min. All AZO thin films showed (002) orientation. For electrical and optical measurements, the films deposited on glass substrates were post-annealed at 400 °C for 30 min in forming gas (100 % H2) to improve their conductivity. These samples had high transparency in the visible wavelength range, and also showed good conductivity. A 0.2 mol L−1 AZO solution with 3 at.% Al content was heated in a Teflon autoclave at 160 °C for 30 min to form AZO nanocrystals, and then the AZO nanocrystals were suspended in the MEA and methanol to obtain the stable AZO colloid. The Al content in the AZO nanocrystals was 2.7 at.%, and the high Al doping coefficient was mainly attributed to the formation of AZO nanocrystals in the autoclave. The AZO thin film using this colloid had the lowest resistivity of 3.89 × 10−3 Ω cm due to its high carrier concentration of 3.29 × 1020 cm−3.

Al-doped ZnO nanocrystals: Electronic states through scanning tunneling spectroscopy

We form a monolayer of undoped and Al-doped ZnO nanocrystals and measure tunneling current with a scanning tunneling microscope tip. From the density of states, we determine the location of conduction and valence band edges with respect to the Fermi energy. We show that with n-doping, Fermi energy of ZnO nanoparticles shifts toward the conduction band edge. The difference between the electronic states, that is, the bandgap of the nanocrystals does not change upon doping. This is in agreement with the optical absorption spectra of the nanomaterials. We also find that inhomogeneity of doping in nanoparticles is reflected in density of states. With an increase in doping concentration, the distribution of dopants among particles becomes broader. We characterize the monolayers also with Hg electrodes to comment on electrical conductivity versus doping concentration behavior.

Structure and Properties of Doped ZnO Nanopowders Synthesized by Methanol Alcoholysis Method

Pured ZnO, Al doped ZnO and Al-In co-doped ZnO nanopowders were synthesized by the methanol alcoholysis method at 130 °C. Structure, morphology and optical properties of ZnO nanopowders were characterized using X-ray diffraction, Transmission electron microscopy (TEM), Fourier transform infrared (FTIR) and Photoluminescence (PL) spectra. The results show that ZnO nanopowders can be obtained in methanol solution at low temperature (130 °C). TEM images show that Al doped ZnO nanocrystals grow along the [002] axis quicker than other axes. FTIR spectra show that ZnO nanocrystals synthesized by the methanol alcoholysis include a little organic impurity. PL spectrums reveal that pure ZnO and doped ZnO nanocrystals have a blue band emission at 440 nm and a green band emission at 520 nm and 530 nm, respectively. Compared with the pure ZnO nanocrystal, the Al doping improves the luminescent properties.

Spin-dependent recombination of defects in bulk ZnO crystals and ZnO nanocrystals as studied by optically detected magnetic resonance

Abstract Long-lasting (several hours) afterglow at helium temperatures was observed in ZnO nanocrystals, 36 nm in size, after switching off the UV excitation. A quenching of the afterglow intensity in the magnetic field was found, which appears because of the preferential orientation of the recombining spins along the field and the spin-dependence of the recombination. ODMR was detected as resonance increase in the afterglow intensity at saturation of the EPR transitions of defects, which take part in the recombination. Shallow donors and different types of acceptors responsible for the afterglow in Al doped ZnO nanocrystals were identified. ODMR of donors and Li acceptors was recorded via the afterglow of bulk ZnO:Li, which could last for about an hour. In the photoluminescence-detected ODMR of bulk ZnO:Li crystals exchange-coupled donor–acceptor pairs were observed in addition to the Li acceptor and donor signals. Exchange-coupled pairs dominated the ODMR spectra recorded on the photoluminescence of ZnO nanocrystals.

Al-doped ZnO nanocrystals

Al 3 + -doped ZnO nanocrystals were differently obtained by wet chemical and an electrochemical route. An increase in forbidden gap due to change in crystal size and also due to Al3+ doping in ZnO is critically analyzed. The Moss–Burstein type shift in Al3+-doped ZnO nanocrystals provides an evidence of successful Al3+ doping in ZnO nanocrystals. The possibility of varying the carrier concentration in ZnO nanocrystals is the indirect implication of the present investigations.

Identification of shallow Al donors in Al-doped ZnO nanocrystals: EPR and ENDOR spectroscopy

Electron paramagnetic resonance and electron-nuclear double resonance (ENDOR) experiments on ZnO nanoparticles doped with Al reveal the presence of shallow, effective-masslike donors related to substitutional Al atoms. The shallow character of the Al donor is evidenced by the multitude of ENDOR transitions of the $^{67}\text{Z}\text{n}$ nuclear spins and by the hyperfine interaction of 1.45 MHz of the $^{27}\text{A}\text{l}$ nuclear spin, which is much smaller than for atomic aluminum. The electric-field gradients at the Al and Zn atoms are found to be nearly the same, which supports the contention that the Al atom is located at a Zn position.