Revista de Ciencias Tecnológicas (RECIT). Volumen 3 (1): 10-22

Revista de Ciencias Tecnológicas (RECIT). Universidad Autónoma de Baja California ISSN 2594-1925

Volumen 7 (4): e375. Octubre-Diciembre, 2024. https://doi.org/10.37636/recit.v7n4e375

1 ISSN: 2594-1925

Research article

Optical and Methanol Sensing Properties of Al-doped ZnO Thin

Film

Propiedades de detección óptica y de metanol de una película delgada de

ZnO dopada con Al

Sumitra Pandey1* , Samundra Marasini1, Rishi Ram Ghimire2

1Goldengate International College, Tribhuvan University, Kathmandu, Nepal

2Department of Physics, Patan Multiple Campus, Tribhuvan University, Patandhoka, Lalitpur, Nepal

*Corresponding author: Sumitra Pandey, Goldengate International College, Tribhuvan University, Kathmandu, Nepal. E-

mail: sumitrapandey12@gmail.com. ORCID: 0009-0008-9627-7595.

Received: September 19, 2024 Accepted: November 19, 2024 Published: November 20, 2024

Abstract.- The study investigates the optical and electrical properties of undoped and aluminum (Al)-doped zinc oxide (ZnO)

films, focusing on their performance as gas sensors and their potential applications. Optical analysis, conducted using UV-visible

spectrophotometry, reveals that 1% Al-doped ZnO films exhibit the highest transmittance of 91%, indicating superior optical

clarity and suitability for applications like solar cell electrodes. In contrast, 3% Al-doped ZnO films show significantly lower

transmittance due to increased light scattering and photon absorption. The bandgap of ZnO films decreases with higher Al doping

concentrations, from 3.3 eV for undoped ZnO to 3.15 eV for 3% Al-doped ZnO, suggesting enhanced electrical conductivity due

to reduced bandgap. The extinction coefficient data demonstrate that 2% Al-doped ZnO has the highest extinction coefficient,

reflecting improved light absorption and scattering properties. Electrical characterization through I-V curves indicates that 1%

Al-doped ZnO films have higher current (121 µA) compared to undoped (431 µA) and higher doping concentrations, attributed

to enhanced carrier concentration and mobility. Sensitivity tests show that 2.5% Al-doped ZnO films exhibit the highest sensitivity

to methanol vapor, with a significant reduction in resistance compared to 0.5% Al-doped ZnO films. Resistance measurements

with varying methanol volumes reveal a rapid decrease upon gas introduction, stabilizing and then increasing as the gas is

removed. Sensitivity analysis indicates that 100 µL methanol provides the highest sensitivity (97%) at 60°C, while 2% Al-doped

ZnO films show consistent sensitivity at 60 °C and 100 °C, but not at 80 °C.

Keywords: ZnO films; Aluminum doping; Optical transmittance; Bandgap reduction; Electrical conductivity; Gas sensor

sensitivity.

Resumen. - El estudio investiga las propiedades ópticas y eléctricas de películas de óxido de zinc (ZnO) dopadas con aluminio

(Al) y sin dopar, centrándose en su rendimiento como sensores de gas y sus posibles aplicaciones. El análisis óptico, realizado

mediante espectrofotometría UV-visible, revela que las películas de ZnO dopadas con Al al 1 % exhiben la transmitancia más

alta del 91 %, lo que indica una claridad óptica superior y su idoneidad para aplicaciones como electrodos de células solares.

Por el contrario, las películas de ZnO dopadas con Al al 3 % muestran una transmitancia significativamente menor debido al

aumento de la dispersión de la luz y la absorción de fotones. La brecha de banda de las películas de ZnO disminuye con mayores

concentraciones de dopaje de Al, de 3,3 eV para ZnO sin dopar a 3,15 eV para ZnO dopado con Al al 3 %, lo que sugiere una

conductividad eléctrica mejorada debido a la brecha de banda reducida. Los datos del coeficiente de extinción demuestran que

el ZnO dopado con Al al 2 % tiene el coeficiente de extinción más alto, lo que refleja propiedades mejoradas de absorción y

dispersión de la luz. La caracterización eléctrica a través de curvas I-V indica que las películas de ZnO dopadas con Al al 1 %

tienen una corriente más alta (121 µA) en comparación con las no dopadas (431 µA) y mayores concentraciones de dopaje,

atribuidas a una mayor concentración y movilidad de portadores. Las pruebas de sensibilidad muestran que las películas de

ZnO dopadas con Al al 2,5 % exhiben la mayor sensibilidad al vapor de metanol, con una reducción significativa en la resistencia

en comparación con las películas de ZnO dopadas con Al al 0,5 %. Las mediciones de resistencia con volúmenes de metanol

variables revelan una rápida disminución al introducir el gas, estabilizándose y luego aumentando a medida que se elimina el

gas. El análisis de sensibilidad indica que 100 µL de metanol proporciona la mayor sensibilidad (97 %) a 60 °C, mientras que

las películas de ZnO dopadas con Al al 2 % muestran una sensibilidad constante a 60 °C y 100 °C, pero no a 80 °C.

Palabras clave: Películas de ZnO; Dopaje de aluminio; Transmitancia óptica; Reducción de la banda prohibida; Conductividad

eléctrica; Sensibilidad del sensor de gas.

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1. Introduction

Zinc oxide (ZnO) is a widely used

semiconducting material known for its ease of

synthesis, cost-effectiveness, non-toxicity,

transparency, and high electron mobility of 2000

cm²/(Vs) at 80 K. It typically crystallizes in the

wurtzite phase and has a direct bandgap of 3.37

eV, with a high exciton binding energy of 60

meV, allowing for efficient excitonic emission at

room temperature [1]. ZnO's various

nanostructures, including films, nanowires,

nanorods, and nanoparticles, are suitable for

applications in sensors, detectors, and thin-film

transistors, with nanostructured thin films

particularly valuable for studying electrical,

thermal, and optical properties [2]. The material's

carrier transport behavior is influenced by light

and other sensing materials, making it attractive

for solar cells, luminescent devices, electrical

and acoustic devices, and chemical sensors [3].

Al-doped ZnO thin films exhibit excellent optical

and electrical properties, such as high electron

mobility, uniformity, and transparency to visible

light, positioning them as promising materials for

next-generation flat panel displays [4]. These

films also possess a broad sensing spectrum

(200-300 nm), making them suitable for UV light

applications, including solar UV radiation

monitoring and ultra-high temperature flame

detection, as well as potential use in transparent

conducting oxide (TCO) electrodes and light-

emitting diodes (LEDs) [5]. The material's

optical absorption is linked to electron transitions

from the valence band to the conduction band and

defect levels, enhancing conductivity through the

desorption of surface oxygen by photogenerated

holes.

Additionally, Zhu et al. [6] developed a novel gas

sensor with high response and selectivity using

molecularly imprinted powders (MIPs). The

sensor demonstrated excellent gas-sensing

properties to methanol vapor, particularly with a

methanol-to-methyl acrylic acid molar ratio of

1:4. At an optimal operating temperature of

130°C, the sensor showed a response of 41 to 1

ppm methanol, with response and recovery times

of 40 seconds and 50 seconds, respectively.

Mingzhi Jiao's research found that ZnO

nanowires, synthesized at 90°C with low

precursor concentration, show better nitrogen

dioxide selectivity compared to other gases, with

higher stability at 600°C [7]. Alaa's study on ZnO

and Al-doped ZnO thin films revealed that

increased Al doping reduced lattice parameters

and bandgap energy [8]. Anandh discovered that

Al doping alters ZnO thin films' structural and

optical properties, increasing the bandgap up to

3% doping before it decreases [9] (Anandh et al.,

2018). Aydın et al. [10] noted that Al doping in

ZnO thin films enhances their suitability for

ammonia gas detection. Kathwate's work

demonstrated that Al doping decreases the

bandgap of ZnO films and improves ammonia

gas sensing [11]. Dubey found that higher Al

doping in ZnO thin films enhances humidity

sensor sensitivity [12]. Khojier showed that Al-

doped ZnO thin films optimize formaldehyde

sensitivity at 2 % Al [13]. Finally, Gulec reported

that 20% Al-doped ZnO films exhibit superior

photocatalytic performance post-annealing,

despite their unique p-type characteristics [14].

The motivation for this research stems from the

increasing demand for efficient, cost-effective,

and environmentally friendly gas sensors,

particularly for detecting hazardous gases like

methanol vapor. Methanol is widely used in

various industries but poses significant health

and environmental risks due to its toxicity,

making its detection critical. Al-doped ZnO thin

films are promising materials for gas sensing

applications due to their superior optical and

electrical properties, including high

transparency, tunable bandgap, and enhanced

carrier mobility. Previous studies have

demonstrated that Al doping improves ZnO’s

sensitivity and selectivity for various gases, such

as ammonia and formaldehyde, by altering its

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bandgap and structural properties. However,

limited research exists on the optimization of Al-

doped ZnO thin films specifically for methanol

vapor detection. The purpose of this research is

to investigate the optical properties of Al doped

ZnO thin film and methanol sensing properties of

Al-doped ZnO thin films with varying Al

concentrations, focusing on their performance as

methanol vapor sensors. This study's significance

lies in its potential to contribute to the

development of highly sensitive, low-cost, and

reliable methanol sensors for industrial safety,

environmental monitoring, and other

applications requiring accurate detection of toxic

gases.

2. Materials and methods

2.1. Synthesis of aluminum doped zinc oxide

films

Aluminum-doped zinc oxide (Al-doped ZnO)

films are highly sought after due to their

multifunctional properties, including

piezoelectric, electrical, optical, and thermal

characteristics. These films are particularly

useful in applications such as gas sensors,

ultrasonic oscillators, and transparent electrodes

in solar cells. The effectiveness of Al-doped ZnO

films largely depends on their microstructure and

surface nanochemistry. Aluminum doping results

in a reduced material density and smaller grain

size, enhancing the electrical conductivity,

magnetic performance, and optical transparency

of ZnO. Extrinsic doping with elements like

aluminum, indium, gallium, copper, or cadmium

is a common method to improve these properties,

as it can induce defects in the ZnO lattice and

widen the bandgap. Al-doped ZnO is especially

valuable in the fabrication of optoelectronic

devices, heterojunctions, superlattices, and

detectors. The sol-gel method, known for its low

cost and ease of composition control, is

frequently used to synthesize these films,

offering benefits such as precise size control, low

processing temperatures, and the production of

cost-effective semiconducting materials.

2.2. Substrates Cleaning

Before depositing the films, the glass slides were

cleaned with diluted HCl, boiled in acetone, and

then dried completely using a dryer and cotton.

Once fully dry, the slides were prepared for

coating.

2.3 . Preparation of ZnO thin films

To prepare the undoped ZnO precursor solution,

2.74 grams of zinc acetate (ZnC4H6O4) were

dissolved in 25 ml of propanol. This mixture was

then stirred at 60°C for 30 minutes using a

magnetic stirrer, resulting in a curdy mixture.

Diethylamine (C4H11N) was then added drop by

drop while continuing to stir until a clear, water-

like solution was formed. The prepared solution

was stored in a cool, dark place for 24 hours

before use. The spin coating technique was

employed to deposit thin films, as thin as 10 nm,

onto flat substrates. This method involves

placing a liquid on a rotating substrate, with the

material deposited at the center either manually

or robotically. The uniformity of the film

depends on the balance between centrifugal and

viscous forces, which are influenced by the spin

speed, solution viscosity, and spinning time.

Typically used for transparent oxide thin films,

this method ensures the production of uniform

and repeatable films. To achieve consistent

deposition, the spin speed and time (usually 30-

45 seconds) were kept constant. For the

preparation of Al-doped ZnO, 3.0178 grams of

aluminum tetrachloride (AlCl3) were mixed with

25 ml of the previously prepared ZnO solution to

achieve a 0.5M concentration. Five different Al-

doped ZnO samples with varying concentrations

(0.5%, 1%, 1.5%, 2%, and 2.5%) were prepared

using the spin coating technique. During this

process, the slides were placed in the spinner, and

the solution of zinc acetate and propanol was

added dropwise (2 drops per cycle). After each

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coating, the slides were heated to 300°C. This

process was repeated for 10 to 15 coats which is equivalent to 0.001 mm to 0.009 mm thickness

the film's resistance fell within the  range.

Figure 1. Flowchart for preparation of ZnO film.

2.4. Fabrication of methanol gas sensor and

gas sensing setup

A cubical aluminum box with each side

measuring 10 mm was constructed, featuring

three air-tight holes sealed with rubber corks.

These holes served different purposes: one for

the methanol gas inlet, another for the outlet, and

the third for the wire connections. The ZnO

sample was placed on the upper side of the box,

and a uniform heat source was applied

consistently. The experiment involved varying

the internal temperature of the box across three

levels. Initially, the temperature was set to 60°C,

and varying volumes of methanol (100µl, 200µl,

300µl, and 400µl) were introduced using a

micropipette. The procedure was repeated at

80°C and 100°C, with the same methanol

volumes being tested at each temperature. The

experimental setup for gas sensing is depicted in

Figure 2.

Figure 2. Experimental setup for gas sensing.

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The heat inside the box facilitated the

vaporization of the methanol. A rheostat was

used to adjust the resistance, and the voltage

supply was managed to maintain the desired

temperature. Together, these components

allowed precise control over the temperature

within the box. The setup featured three holes on

the ceiling of the box, all sealed with rubber

corks. One cork served as the inlet for the gas,

introduced via a syringe, while another cork was

left open as the outlet. The third hole

accommodated the connections for the

multimeter, rheostat, and voltage supply.

Additionally, a heater was positioned at the

bottom of the box, with the ZnO film placed on

the ceiling.

2.5. Mechanism of Conductivity

The mechanism of conductivity in Al-doped ZnO

thin films is primarily influenced by the

incorporation of aluminum ions into the ZnO

crystal lattice, which alters its electrical

properties. ZnO is an intrinsic n-type

semiconductor, primarily due to oxygen

vacancies and zinc interstitials that provide free

electrons. When aluminum is doped into ZnO,

the Al³⁺ ions, having a smaller ionic radius and

higher valency than Zn²⁺ ions, replace some Zn²⁺

in the lattice. This substitution creates extra free

electrons because Al³⁺ donates one more electron

than Zn²⁺, thereby enhancing the electron carrier

concentration, which leads to an increase in

electrical conductivity. Al-doped ZnO

composites exhibit improved charge storage

behavior due to their enhanced carrier mobility

and increased electron density. The higher

concentration of free electrons in Al-doped ZnO

enables greater polarization under an external

electric field, which enhances the dielectric

properties and thus the material's ability to store

charge. This is especially relevant in dynamic

electrical applications, such as capacitors and

sensors, where the charge storage capacity plays

a crucial role.

2.6. Characterization of ZnO films

UV-Visible spectroscopy is used to analyze

materials by measuring their absorbance,

reflectance, or transmittance of light in the UV to

visible range, where electronic transitions occur

in molecules. According to the Beer-Lambert

law, absorbance is directly proportional to both

the concentration of the absorbing species and

the path length. This method is useful for

determining concentration and calculating the

optical band gap of thin films [15].

  󰇡

󰇢   (1)

A represents the measured absorbance, I0 is the

intensity of the incident light, I is the transmitted

intensity, L denotes the path length through the

sample, and c is the concentration of the

absorbing species. Additionally, the band gap of

a thin film can be calculated using UV-Visible

spectroscopy:

󰇛󰇜  󰇛  󰇜 (2)

The optical band gap (Egap) of a semiconductor

can be determined using UV-Visible

spectroscopy. Here, hν represents the incident

photon energy, B is a constant, and m is 0.5 for

direct and 1 for indirect band gaps. The

absorption coefficient (α(ν)) is calculated using

Beer-Lambert's law: α(ν)= (2.303Abs(λ))/d,

where Abs(λ) is the absorbance and d is the film

thickness. When light interacts with a

semiconductor, it can be absorbed, transmitted,

or reflected. The energy required for electron

transitions between the valence and conduction

bands varies with different materials. In direct

optical absorption, the conduction and valence

band edges align at the same wave vector (k),

allowing electrons to transition directly from the

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valence band to the conduction band without a

change in momentum when hν>Eg [16-17].

3. Result and discussion

3.1. Optical properties

Transmittance, which measures the fraction of

incident light passing through a substance, was

analyzed for ZnO films using a UV-visible

spectrophotometer. This analysis compared the

optical transmittance of undoped ZnO films with

those doped with varying concentrations of

aluminum. The thickness of the films was kept

constant (0.001 mm) to accurately determine the

absorption coefficient and evaluate the optical

properties. The transmittance of 1% Al-doped

ZnO films was found to be 91%, indicating a high

level of optical clarity on figure 3a. This high

transmittance suggests that the 1% Al doping

enhances the crystalline quality of the films,

making them suitable for applications such as

solar cell electrodes. The transmittance spectra

for these films displayed interference fringes,

which are indicative of high surface quality.

These fringes result from reflections at the film

surface with minimal absorption and scattering in

the bulk, further confirming the excellent quality

of the 1% Al-doped ZnO films [18].

Figure 3. (a) Transmittance with 1% Al doped for TM(Tmax) and Tm (Tmin), and (b) transmittance of undoped, 1%, 2%, 3% Al

doped ZnO.

In contrast, the 3% Al-doped ZnO films

exhibited a much lower transmittance of 11%,

which increased to 60% with varying methanol

volumes. This significant reduction in

transmittance is attributed to the increased

doping concentration, which introduces higher

charge density and consequently enhances

photon absorption. The presence of defect sites

from higher doping levels leads to increased light

scattering, which degrades the optical

performance of the films as shown in figure 3b.

This scattering effect is evident from the lower

transmittance values observed for the 3% Al-

doped ZnO films [18]. The transmittance of 2%

Al-doped ZnO films was recorded at 74%. This

value is intermediate between the 1% and 3%

doping levels, reflecting a balance between

improved crystalline quality and the introduction

of defect sites. The decrease in transmittance

compared to the 1% doping level indicates that

while some benefits of doping are retained,

further increasing the concentration introduces

more defects, leading to increased light scattering

and reduced transmittance [19]. Undoped ZnO

films exhibited a transmittance of 62%, which is

lower than that of 1% Al-doped ZnO but higher

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than that of 3% Al-doped ZnO shown in figure 3.

This suggests that while undoped ZnO films have

relatively lower optical clarity compared to the

1% Al-doped films, they perform better than the

higher doping concentrations in terms of light

transmission.

Figure 4a illustrates the variation in the direct

bandgap of ZnO films with increasing Al doping

concentration. The bandgap of undoped ZnO is

measured at 3.3 eV. For 1% Al-doped ZnO, the

bandgap decreases to 3.25 eV. Further increasing

the Al concentration to 2% and 3% results in

bandgaps of 3.2 eV and 3.15 eV, respectively.

This decrease in bandgap with higher doping

concentrations can be attributed to the

interactions between the dopant and the host

material. The reduction in bandgap with

increased Al doping is explained by the

incorporation of Al3+ ions, which introduce

additional electronic states within the bandgap.

Specifically, the 3d levels of the Al3+ ions

interact with the sp-electrons of the ZnO lattice,

resulting in strong sp-d exchange interactions.

This interaction modifies the electronic structure

of ZnO, leading to a narrowing of the bandgap

[20]. As shown in Figure 4b, the bandgap

reduction is linearly related to the Al

concentration, indicating a consistent impact of

doping on the electronic properties of the ZnO

films.

Figure 4: (a) Comparison bandgaps of undoped, 1%, 2%, and 3% Al-doped ZnO and (b) Band gap vs doping concentration

A narrower bandgap generally enhances

electrical conductivity, as it reduces the energy

required for electron excitation from the valence

band to the conduction band. Therefore, the

observed decrease in bandgap with increased Al

doping suggests that the electrical conductivity

of the ZnO films is likely to improve with higher

doping concentrations. The improved

conductivity is beneficial for applications

requiring efficient charge transport, such as in

transparent electrodes and optoelectronic

devices.

Figure 5 presents the extinction coefficient for

ZnO films with varying Al doping

concentrations. Notably, the 2% Al-doped ZnO

sample exhibits a higher extinction coefficient

compared to the 1% Al-doped ZnO sample. The

extinction coefficient is a measure of how much

light is absorbed and scattered by a material,

influencing its dielectric loss. A higher extinction

coefficient indicates greater light absorption and

scattering. The extinction coefficient trends

observed in the samples correlate with their

doping levels. For the 2% Al-doped ZnO, the

extinction coefficient initially decreases and then

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increases with wavelength. This behavior

suggests that the optical properties of the film are

significantly influenced by the doping

concentration. Conversely, undoped, 1%, and 3%

Al-doped ZnO films show a similar extinction

coefficient pattern, indicating a more stable

relationship between wavelength and extinction

coefficient within these samples.

Figure 5. Extinction coefficient of undoped, 1% Al, 2% Al, 3% Al-doped ZnO vs wavelength.

The general trend for all samples shows that the

extinction coefficient decreases with increasing

wavelength. This decrease is attributed to the

scattering of light, which reduces absorbance. As

the wavelength of light increases, the absorption

decreases, leading to a lower extinction

coefficient. This trend is consistent across the

undoped, 1%, and 3% Al-doped ZnO films. The

increase in the extinction coefficient for higher

Al doping levels, specifically for the 2% Al-

doped ZnO, is due to reduced light scattering and

increased absorbance. With more Al doping, the

ZnO film's microstructure becomes more

conducive to absorbing light, leading to higher

absorbance and, consequently, a higher

extinction coefficient. This improvement in

absorbance is linked to the enhanced electronic

interactions introduced by the Al dopants, which

modify the optical properties of the ZnO film.

Figures 6a and 6b illustrate the real and

imaginary parts of the dielectric functions for

ZnO films with varying Al doping

concentrations. The dielectric functions provide

crucial insights into the optical properties of the

films, with the real part 󰇛 󰇜 related to

the refractive index (n) and the imaginary part

( ) associated with the extinction

coefficient (k). The real part of the dielectric

function 󰇛󰇜 is generally higher than the

imaginary part () across all ZnO samples. This

suggests that the films exhibit good transparency,

as a higher  is indicative of a higher refractive

index, which typically corresponds to reduced

light absorption. Among the samples, the 1% Al-

doped ZnO films show a higher  compared to

the 2% Al-doped ZnO films, indicating superior

transparency and optical quality. The real part's

graph, however, exhibits increased noise at

higher doping concentrations, complicating

accurate analysis and interpretation. The

imaginary part of the  reflects the material’s

absorption characteristics. For the ZnO films, 

is generally lower than , signifying relatively

low absorption. Specifically, the 1% Al-doped

ZnO films have a higher  compared to the 2%

Al-doped ZnO films, supporting the notion that

1% Al-doped films exhibit better transparency

and reduced absorption. The data for  shows

consistent behavior across different Al

concentrations, which aids in understanding the

films' absorption properties. Analysis and

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Implications: The analysis of dielectric functions

indicates that the 1% Al-doped ZnO films have

better transparency and lower absorption

compared to films with higher Al doping

concentrations. This suggests that a lower doping

concentration, such as 1%, is advantageous for

applications requiring high optical clarity, such

as transparent electrodes in solar cells. The

observed noise in the real part of the dielectric

function for higher doping levels reflects

challenges in measurement accuracy, likely due

to increased light scattering and other optical

effects. Sensitivity and Application: The

dielectric property findings are crucial for

optimizing ZnO films in gas sensing

applications. The lower absorption and higher

transparency of 1% Al-doped ZnO films could

offer a balance between optical performance and

sensor sensitivity. Conversely, higher doping

levels might alter the sensor response due to

increased absorption. Thus, tailoring the doping

concentration is essential for achieving the

desired balance in various applications.

Figure 6. (a) Real part of dielectric of 1% and 2% Al-doped ZnO vs wavelength, and (b) Imaginary part dielectric of vs

wavelength

3.2. Electrical properties

The gas sensing mechanism in Al-doped ZnO

thin films is primarily based on surface sensing,

where the interaction between the target gas

(methanol vapor) and the film's surface leads to

changes in electrical properties. When exposed to

air, oxygen molecules adsorb onto the ZnO

surface, trapping free electrons from the

conduction band and forming negatively charged

oxygen species (O₂⁻, O⁻). This creates a depletion

layer near the surface, reducing the film's

conductivity.

When methanol vapor is introduced, it reacts

with these oxygen species, releasing the trapped

electrons back into the conduction band, thereby

reducing the depletion layer and increasing

conductivity. The extent of this change in

conductivity depends on the concentration of

methanol vapor and the efficiency of electron

transfer. Al-doping enhances this surface

interaction by increasing the electron density,

which boosts the sensor's sensitivity, allowing it

to detect lower concentrations of methanol with

faster response and recovery times. This surface

reaction mechanism is central to the gas sensing

properties of Al-doped ZnO thin films.

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The I-V curve illustrates the relationship between

the applied voltage and the resulting current in

ZnO films. As depicted in Figure 7a, there is a

clear linear relationship where an increase in

voltage corresponds to an increase in current.

This behavior is typical of conductive materials,

where the current is directly proportional to the

applied voltage, following Ohm's Law. However,

the magnitude of the current varies depending on

the doping concentration. The undoped ZnO film

exhibits a high current of 431 µA at elevated

voltages. This high current is attributed to the

specific dimensions of the sample, which can

affect the current measurement. Undoped ZnO

generally has lower resistivity due to the absence

of doping-induced defects that would otherwise

impede charge carrier movement. In contrast, the

2.5% Al-doped ZnO film shows a significantly

lower current of 17 µA. This reduction is likely

due to the increased defect density and potential

scattering of charge carriers at higher doping

concentrations, which impedes carrier mobility.

The current increases from 69 µA at 0.5% Al

doping to 121 µA at 1% Al doping. This increase

is due to the enhanced carrier concentration

resulting from Al doping. Aluminum introduces

extra electrons into the ZnO lattice, which

increases electrical conductivity by providing

additional charge carriers. Carrier Concentration:

The rise in current with moderate Al doping

(0.5% to 1%) indicates improved carrier

concentration and enhanced electrical

conductivity. As Al concentration increases,

more electrons are introduced into the ZnO

lattice, enhancing carrier mobility and

conductivity. At higher doping levels (e.g., 2.5%

Al), the observed decrease in current can be

attributed to increased defect density. Excessive

doping creates a higher number of scattering

centers and traps, which can hinder carrier

mobility and reduce the overall current. This

phenomenon reflects a common trade-off where

initial increases in carrier concentration improve

conductivity, but excessive doping can lead to

diminished performance due to defect-related

scattering and carrier recombination [22].

Figure 7. (a) Current vs Voltage and (b) Comparison of resistance of undoped, 0.5%Al, 1%Al, 1.5%Al, 2%Al, and 2.5%Al

doped ZnO vs time at 100l of serum at 60 oC

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Figure 7b shows the resistance of ZnO films with

varying Al concentrations over time. The

resistance initially decreases when gas is

introduced, stabilizes for approximately 130

seconds, and then gradually returns to its original

value within 100 seconds. Upon gas introduction,

resistance drops, indicating a quick reaction to

the gas exposure. This decrease is likely due to

the interaction between the gas and the ZnO film,

which alters the carrier concentration or surface

properties temporarily. After the initial drop, the

resistance stabilizes and eventually increases

back to the original value. This behavior suggests

that the gas exposure causes a transient change in

carrier dynamics or surface chemistry, which

later returns to equilibrium. The resistance of

ZnO films decreases with higher Al doping

concentrations. This is because high doping

concentrations lead to increased defect density

by trapping or scattering electrons which lead to

higher resistance over time [23].

Figure 8 shows that sensitivity to methanol vapor

increases with Al doping concentration in ZnO

films, with 2.5% Al-doped ZnO exhibiting the

highest sensitivity and 0.5% Al-doped ZnO the

lowest. Higher doping concentrations introduce

more free electrons into the ZnO lattice, which

enhances the film's response to methanol vapor.

The presence of methanol, a reducing agent,

decreases the resistance of the film by reducing

oxygen species at the surface and grain

boundaries, leading to increased sensitivity.

Specifically, the 2.5% Al-doped ZnO film shows

a significant reduction in resistance due to a

higher concentration of free electrons and more

active reaction sites, whereas the 0.5% Al-doped

ZnO film, with fewer free electrons and reaction

sites, demonstrates lower sensitivity. Thus,

optimizing Al doping levels improves the

sensitivity of ZnO films for gas detection

applications. Also, Al-doped ZnO thin films

offer high sensitivity, fast response, and recovery

times, making them ideal for developing

efficient, low-cost sensors for real-time methanol

vapor monitoring. This aligns with the need for

accurate, portable sensors in industrial and

environmental applications, ensuring safe

handling and compliance with regulations.

Figure 8. Comparisons of sensitivity of undoped, 0.5% Al, 1.5% Al, 2% Al, 2.5% Al-doped ZnO vs Time at 100 l and 60 oC.

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Figure 9 illustrates the resistance response of

ZnO films to various volumes of methanol vapor.

The graph shows that resistance decreases

rapidly upon introducing methanol vapor, with

the greatest reduction occurring in the first few

seconds. This decrease in resistance stabilizes

around 180 seconds before starting to rise as the

methanol vapor is removed. Notably, the 200 µL

methanol volume exhibits a more pronounced

early increase in resistance compared to other

volumes. Specifically, the resistance drops from

5.8  to 1.4 Ω for 400 µL, from 3.4 Ω to 1.31 Ω

for 300 µL, from 3.1 Ω to 1.0 Ω for 200 µL, and

from 3.5 Ω to 4.0 Ω for 100 µL. The observed

trend across all volumes reflects a similar pattern

of resistance change. This resistance decrease is

attributed to the chemisorption of methanol

molecules on the ZnO surface, leading to the

formation of O₂⁻ ions, which act as electron

acceptors. This process increases the electron

carrier concentration near the surface, enhancing

conduction and reducing resistance [23].

Figure 9. Resistance of undoped ZnO at 100, 200, 300, 400 µl methanol gas at 60oC Vs time.

Figure 10a presents the sensitivity of undoped

ZnO films to methanol vapor at 60 °C across

different methanol volumes (100, 200, 300, and

400 µL). Initially, sensitivity increases rapidly

within the first 50 seconds, remains stable for

approximately 100 seconds, and then begins to

decrease. This behavior is attributed to the high

initial concentration of methanol vapor, which

increases the number of reducing molecules,

thereby enhancing the reduction of oxygen at the

grain boundaries of the ZnO film. This reduction

leads to a significant drop in resistance, resulting

in higher sensitivity. As methanol molecules

evaporate over time, their concentration

decreases, leading to fewer oxygen molecules

being reduced and causing the resistance to rise.

Consequently, the sensitivity decreases as the

methanol concentration diminishes. Among the

different volumes tested, 100 µL methanol

exhibits the highest sensitivity at 97%, indicating

that lower methanol concentrations yield better

sensitivity in this setup.

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Figure 10. (a) Sensitivity of undoped ZnO with 100, 200, 300, 400 µl methanol gas at 600°C temperature vs time and

(b)Sensitivity of 2% Al-doped ZnO of 100 µl methanol gas at 60°C, 80°C and 100°C temperature vs time.

Figure 10b explores the sensitivity of 2% Al-

doped ZnO films to 100 µL methanol vapor at

different temperatures: 60 °C, 80°C, and 100 °C.

At both 60 °C and 100 °C, sensitivity initially

increases rapidly, remains stable for a brief

period, and then starts to decline. This pattern

suggests that the sensor responds well to

methanol vapor at these temperatures, with a

consistent initial response followed by a gradual

decrease as the methanol vapor disperses.

Conversely, at 80 °C, sensitivity peaks abruptly

and then drop sharply without maintaining a

stable phase. This sudden decline may be due to

the higher temperature accelerating the

evaporation rate of methanol and possibly

affecting the adsorption dynamics on the ZnO

surface, leading to a less stable sensitivity

response.

4. Conclusion

The results demonstrate that doping ZnO with

aluminum significantly impacts its optical and

electrical properties. The optimal doping

concentration for maintaining high optical

transmittance and improving electrical

conductivity is 1%. Higher doping

concentrations, while enhancing sensitivity to

methanol vapor, introduce more defect sites and

reduce optical clarity. Specifically, 2% Al-doped

ZnO films offer a good balance between high

sensitivity and reasonable optical properties.

Sensitivity to methanol vapor is maximized with

lower methanol concentrations (100 µL) and at

specific temperatures (60 °C and 100 °C),

indicating that fine-tuning doping levels and

operational conditions can optimize ZnO-based

sensors for various applications.

5. Authorship acknowledgements

We would like to express our sincere gratitude to

Goldengate International College, Tribhuvan

University, Kathmandu, Nepal, for providing us

with the necessary laboratory facilities to carry

out this research. Additionally, we extend our

thanks to the Department of Physics, Patan

Multiple Campus, Tribhuvan University,

Patandhoka, Lalitpur, Nepal, for their valuable

motivational support throughout this study.

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