Al‑Doped ZnO Nanospears: Hydrothermal Growth, Microstructure, and Temperature‑Dependent Photoluminescence

Abstract

Al‑doped ZnO (AZO) nanospears were synthesized by a hydrothermal route on ZnO‑seeded glass substrates. X‑ray diffraction confirms a hexagonal wurtzite lattice with a strong (002) orientation, while scanning electron microscopy shows highly aligned, needle‑shaped structures with fine tips. Increasing Al concentration shortens the nanospears, a trend mirrored in the XRD peak intensities due to pH changes during growth. Photoluminescence (PL) at room temperature reveals a near‑band‑edge (NBE) emission at ~3.16 eV and a violet emission (VE) at ~2.91 eV that intensify with Al content, whereas the deep‑level emission (DLE) remains largely unaffected. Variable‑temperature PL down to 10 K uncovers an additional excitonic peak at ~3.31 eV and fine structure below 57 K; the NBE emission quenches above 267 K, indicating defect‑mediated thermal quenching.

Background

One‑dimensional ZnO nanostructures are pivotal for next‑generation optoelectronics due to their wide bandgap (3.37 eV) and high exciton binding energy (60 meV). Doping with group III elements (B, Al, Ga, In) introduces shallow donors, enabling n‑type conductivity and enhanced transparency. Among them, Al is favored for its small ionic radius, low cost, and ability to modulate optical and electronic properties. While hydrothermal synthesis offers low‑temperature, scalable production, most studies focus on morphology and electrical behavior; temperature‑dependent PL of AZO nanospears remains underexplored.

Methods

Sample Preparation

Glass substrates were coated with a ZnO seed layer via a sol‑gel process (zinc acetate, ethylene glycol, mono‑ethanolamine). After spin‑coating and annealing at 500 °C, the seeded substrates were immersed in a Teflon‑lined autoclave containing zinc nitrate, aluminum nitrate (0–0.12 M), hexamethylenetetramine, and aqueous ammonia (pH ≈ 10). Hydrothermal growth proceeded at 368 K for 1 h, yielding nanospears with lengths decreasing from 1370 nm (0 M) to 740 nm (0.12 M).

Characterization

Crystal structure and morphology were examined by XRD (MXP18AHF) and FE‑SEM (S‑4800). Lengths were measured with a surface profiler (XP‑1). Elemental composition was determined by XPS (ESCALAB 250). PL spectra were recorded with a Horiba Jobin Yvon iHR320 spectrograph, excited by a 325‑nm He‑Cd laser, across 10–297 K.

Results and Discussion

Microstructure and Morphology

XRD patterns confirm a single‑phase wurtzite structure with preferential (002) growth; peak intensities diminish with higher Al, attributed to reduced pH (10.16 → 9.60) that hampers ZnO crystallization. SEM images (Fig. 3) reveal hexagonal, needle‑shaped nanospears (~100 nm diameter) with sharp tips; Al doping shortens the average length in proportion to dopant concentration, consistent with the XRD trend.

XRD patterns of AZO nanospears

Average length of AZO nanospears

SEM images of 0 M ZnO and 0.08 M AZO nanospears: (a) top view, (b) side view of 0 M ZnO, (c) top view, (d) side view of 0.08 M AZO

Compositions

XPS confirms Al incorporation into the ZnO lattice. For 0.12 M AZO, the Al 2p peak at 73.9 eV indicates Al–O bonds; the O 1s spectrum shows peaks at 530.28, 531.41, and 532.26 eV, attributable to O–Zn, O–Al, and C–O, respectively. The Al content is ~1.29 % at the surface, while Zn/O ratios remain close to stoichiometry.

XPS spectra of 0.12 M AZO nanospears: (a) survey, (b) Zn 2p, (c) O 1s, (d) Al 2p

Photoluminescence Properties

Room‑temperature PL (Fig. 5) shows two dominant peaks: a VE (~2.91 eV) and an NBE (~3.16 eV) that intensify with Al content, while the DLE (~2.2–2.4 eV) remains unchanged. Peak fitting (Fig. 6) reveals that undoped ZnO lacks the VE component, suggesting that Al introduces the shallow donor level responsible for VE. The NBE shift is consistent with donor‑acceptor pair recombination enhanced by increased donor concentration.

Room‑temperature PL spectra of AZO nanospears. The bottom spectrum is the undoped ZnO reference.

Peak fitting of PL spectra: (a) 0 M ZnO, (b) 0.04 M AZO, (c) 0.08 M AZO, (d) 0.12 M AZO

Variable‑temperature PL (Fig. 7) demonstrates that the VE and NBE peaks remain largely temperature‑independent, whereas the DLE intensity drops rapidly with increasing temperature, reflecting the activation of non‑radiative pathways. Below 57 K, the NBE exhibits fine structure: a peak at 3.33 eV (donor‑bound exciton) and a weaker 3.37 eV (free exciton). These features vanish above 267 K, indicating defect‑mediated exciton scattering and thermal quenching.

Temperature‑dependent PL spectra of 0.08 M AZO nanospears.

Peak fitting of PL spectra of 0.08 M AZO nanospears at (a) 10 K, (b) 117 K, (c) 207 K, (d) 267 K.

Conclusions

Hydrothermal synthesis yields Al‑doped ZnO nanospears that grow along the c-axis with sharp tips. Al incorporation shortens the nanospears and introduces a VE band that, together with the NBE emission, scales with dopant level. The DLE remains temperature‑sensitive, while the NBE excitonic peak displays fine structure at low temperatures and quenches above 267 K due to defect‑mediated exciton scattering. These findings elucidate the role of Al and temperature on the optical behavior of ZnO nanostructures, informing future optoelectronic device design.

Abbreviations

AZO

Al‑doped ZnO

DLE

Deep level emission

DX

Donor‑bound exciton

FX

Free exciton

NBE

Near band edge

PL

Photoluminescence

SEM

Scanning electron microscopy

VE

Violet emission

XPS

X‑ray photoelectron spectroscopy

XRD

X‑ray diffraction