Enhancing ZnO Nanowire Optoelectronics: Ar Plasma Treatment Boosts Emission and Enables Room‑Temperature Lasing

Introduction

Zinc oxide (ZnO) remains a cornerstone semiconductor for light‑emitting, photodetector, and piezoelectric applications due to its intrinsic wide bandgap and strong excitonic effects. One‑dimensional nanowires amplify these properties, achieving the first optically pumped ZnO nanowire laser and demonstrating high‑performance field‑effect transistors without external doping. However, the high surface‑to‑volume ratio inherent to nanowires introduces surface trap states (SS) and adsorbed species that severely degrade optical output. Therefore, surface engineering is essential for harnessing the full potential of ZnO nanowires.

Multiple surface modification strategies—metal coating, core‑shell architectures, polymer encapsulation, and plasma‑assisted etching—have been explored. Among them, Ar plasma etching stands out for its simplicity, safety, and cost‑effectiveness. While prior studies have employed various plasma gases (H₂, Ga⁺, CH₄, Ar), the impact of plasma energy on ZnO nanowires remains underexplored. This work systematically investigates Ar plasma treatment at different RF powers to elucidate how energy influences surface chemistry, morphology, and optical performance.

Methods

Preparation of ZnO Nanowires

ZnO nanowires were grown by the vapor‑liquid‑solid method. A 1:1 mass ratio of ZnO to graphite powders served as the source material, loaded into a quartz boat. A 3 nm Au film sputtered on sapphire acted as the catalyst. The tube furnace temperature sequence was 200 °C (50 °C/min), 700 °C (50 °C/min, 15 min), then 950 °C (50 °C/min) while flowing Ar (99 mL/min) and O₂ (1 mL/min) for 30 min. Cooling under Ar yielded nanowires with ~170 nm diameter. Samples were divided into six batches for subsequent treatments.

Ar Plasma Treatment

Plasma processing employed a Sentech SI‑500 ICP system with PTSA200 source. ICP power was fixed at 180 W; RF power varied from 0 to 400 W to control ion energy. Ar flow was 25 SCCM at 1 Pa pressure. Each batch was exposed for 90 s at 25 °C.

Morphology Characterization and Photoluminescence Measurements

Field‑emission SEM (Hitachi‑4800) examined morphology. Temperature‑dependent PL (50–300 K) used a 325 nm He‑Cd laser (2 mW, 0.4 cm² spot). For high‑density excitation, a pulsed Nd:YAG 266 nm laser (1 ns, 60 Hz, 3 × 10⁻⁴ cm²) was employed. Emission was dispersed by an Andor SR‑500 monochromator and recorded with a UV‑enhanced CCD.

Results and Discussion

SEM images (Fig. 1) show pristine nanowires (~170 nm) with a clear prismatic cross‑section. Low‑energy plasma (0 W) slightly roughens the surface without altering the shape. Increasing RF power to 200 W transforms the cross‑section to a smooth circle, indicating effective surface cleaning and possible defect passivation. At 400 W, the nanowires fracture, confirming that excessive ion energy causes irreversible damage.

SEM image of the ZnO NWs irradiated by Ar plasma with different energies. a As‑grown. b 0 W. c 200 W. d 400 W

Room‑temperature PL spectra (Fig. 2a) reveal a dramatic intensity increase, peaking at 200 W RF power—a 60‑fold boost relative to as‑grown samples. The full width at half maximum (FWHM) narrows with increasing power up to 200 W, then broadens again at 400 W, mirroring the morphological transition. This behavior reflects the interplay between surface cleaning, defect reduction, and damage accumulation.

a Room‑temperature PL spectra of the ZnO NWs treated by Ar plasma with different energies (inset shows the repeatability of this treatment). b Integrated intensity and FWHM versus plasma energy. c, d Schematic band structure of As‑grown sample and after plasma‑treated sample

Low‑temperature PL (Fig. 3) identifies a dominant D⁰X peak at 3.363 eV, with weaker free‑exciton (FX) and donor‑acceptor pair (DAP) features in the as‑grown and 0 W samples. After 200 W treatment, the DAP signature disappears, leaving a sharp D⁰X‑like peak at 3.361 eV. The narrow line and absence of FX suggest that Ar plasma has removed acceptor states while enriching neutral donor levels, thereby suppressing non‑radiative pathways.

Low‑temperature PL spectra of the ZnO NWs treated by Ar plasma with different energies. a As‑grown. b 0 W. c 200 W

Temperature‑dependent PL (Fig. 4) further confirms the dominant D⁰X emission in the 200 W sample, which red‑shifts with temperature, following the Bose‑Einstein relation for excitonic transitions. In contrast, the as‑grown sample shows a gradual decline of D⁰X intensity, with FX persisting across the entire temperature range. These observations underscore the efficacy of moderate‑energy Ar plasma in stabilizing excitonic recombination.

a, b Temperature‑dependent PL spectra of the As‑grown ZnO NWs and irradiated by 200 W Ar plasma. c Photon energy and PL emission from the as‑grown sample

Under high‑density optical pumping (266 nm, 1 ns, 60 Hz), the 200 W‑treated nanowires exhibit clear lasing at 390 nm when the pump energy exceeds ~25 µJ, evidenced by sharp spectral peaks and nonlinear intensity growth (Fig. 5). This lasing originates from the P‑band or self‑absorption processes typical of ZnO. The treated samples also display enhanced long‑term stability, with the emission intensity ratio improving over time relative to as‑grown controls (Fig. 5b).

a Lasing under optical pumping from ZnO NWs irradiated by 200 W Ar plasma. b Stability of ZnO NWs (the intensity ratio after plasma treatment over time compared with the as‑grown samples)

Conclusions

We have systematically mapped the influence of Ar plasma energy on ZnO nanowire optical properties. Low‑energy treatment (0–100 W) primarily cleans the surface, modestly enhancing PL. Moderate energy (≈200 W) optimally balances defect passivation and donor activation, yielding a 60‑fold PL increase and enabling room‑temperature lasing. Exceeding ~400 W damages the nanowires and degrades performance. This straightforward, scalable approach offers a powerful tool for improving ZnO‑based optoelectronic devices and advancing ultraviolet photonics.

Availability of Data and Materials

The authors confirm that all materials and data are immediately available to researchers without undue restrictions. All datasets are included within this publication.

Abbreviations

CCD:

Charged coupled device

D⁰X:

Neutral donor‑bound exciton

DAP:

Donor‑acceptor pair

FESEM:

Field‑emission scanning electron microscopy

FWHM:

Full‑width at half maximum

FX:

Free exciton

ICP:

Inductively coupled plasma

LO:

Longitudinal optical

PL:

Photoluminescence

RF:

Radio frequency

SCCM:

Standard‑state cubic centimeters per minute

SS:

Surface trap states