Exploring Electrical Conductivity of Thiolated Micro- and Nanoparticles of Gallium

Nano‐/microparticles of gallium (Ga), as a low‐melting‐point metal, are extensively used in the fields of soft electronics and sensors to provide thermal and electrical conductivity. However, a passivating oxide layer can be formed on the surface of Ga nano‐/microparticles during the synthesis process. This oxide layer is removed by a secondary sintering step, especially mechanical sintering, which is generally not a controllable process, and compromises the integrity of the system. Herein, thiol molecules, 1‐butanethiol, thiophenol, and 4‐mercaptopyridine, that can functionalize the surface of Ga via sonication to reduce the oxidation of Ga surface are used. The resulting particles exhibit electrical conductivity based on metal–molecule junctions without the requirement for a sintering step. In particular, 4‐mercaptopyridine functionalized, thiolated Ga particles exhibit higher electrical conductivity compared to the other three thiolated Ga systems as the organic material conjugation provides conductive pathways for the mix. Subsequently, using these particle systems, soft devices are developed that can be used for gas, exhalation, and flex sensing. This study provides insights into the possibility of creating combinations of organic molecules with liquid metal‐based nano‐/microparticles to generate electrically conductive mixes and the prospects of fabricating multifunctional sensors.

DOI: 10.1002/aisy.202200364 Nano-/microparticles of gallium (Ga), as a low-melting-point metal, are extensively used in the fields of soft electronics and sensors to provide thermal and electrical conductivity. However, a passivating oxide layer can be formed on the surface of Ga nano-/microparticles during the synthesis process. This oxide layer is removed by a secondary sintering step, especially mechanical sintering, which is generally not a controllable process, and compromises the integrity of the system. Herein, thiol molecules, 1-butanethiol, thiophenol, and 4-mercaptopyridine, that can functionalize the surface of Ga via sonication to reduce the oxidation of Ga surface are used. The resulting particles exhibit electrical conductivity based on metal-molecule junctions without the requirement for a sintering step. In particular, 4-mercaptopyridine functionalized, thiolated Ga particles exhibit higher electrical conductivity compared to the other three thiolated Ga systems as the organic material conjugation provides conductive pathways for the mix. Subsequently, using these particle systems, soft devices are developed that can be used for gas, exhalation, and flex sensing. This study provides insights into the possibility of creating combinations of organic molecules with liquid metal-based nano-/microparticles to generate electrically conductive mixes and the prospects of fabricating multifunctional sensors.
problems of such sintering processes. Chemical and electrical sintering have also been suggested for transformation of nonconductive liquid metal mixes into conductive entities. [19] Similarly, these procedures also have their own challenges, loss of integrity and lack of control over the final products.
To achieve electrical conductivity for the Ga-based inks or composites without sintering, several approaches have been demonstrated for the surface functionalization of liquid metals including coatings that participate via physical or chemical interactions with the surface of Ga. [19,20] Mixtures of inorganic particles have been shown to embed themselves into the skin of liquid metal droplets, penetrating into the oxide layer, to provide conductive pathways out of the droplets. [20][21][22] Polymerization on the surface of liquid metals has also been shown to offer the possibilities of surface interaction with precursor monomers during the process. [22][23][24][25] Regarding ligand binding materials, the process involves chemical bonding between liquid metals and small molecules or polymers. Interestingly, thiol molecules have been shown to reduce the Ga oxide formation by 30% during the sonication process of eutectic gallium indium, although the conductivity was not assessed in this work. [26] We hypothesize that functionalization of Ga droplets with conductive molecules, which suppress the surface oxidation, can result in achieving conductive mixes via metal-organic molecules junctions that may not require the sintering steps. Additionally in such a capping process, the integrity of the initial structures can be preserved. The goal of our study is to investigate organic molecules that can functionalize the surface of Ga particles and at the same time, provide electrical conductivity by reducing the possibly of Ga surface oxidation. A conjugated molecule is considered as a factor that can induce the electron flow to create conduction pathways. [27] Therefore, we explored three organic thiol molecules and a reference molecule allyl sulfide with and without conjugation. We compared the conductivity of mixes to gain insight regarding the interfacial properties. A series of proof-of-concept examples are also presented to illustrate the applications of the Ga-based mixes in developing gas, exhalation, and flex sensors.

Synthesis of Thiolated Molecules/Ga Particles
The thiolated Ga particles were synthesized by sonicating bulk Ga droplets in the presence of the thiol molecules in a mixed DMSO/NaOH solution (see Section 4 for details) as schematically presented in Figure 1a. Here, 1-butanethiol, allyl sulfide, thiophenol, and 4-mercaptopyridine were used for the functionalization of liquid Ga (Figure 1b-e). These molecules have different physical and chemical characteristics due to their specific structures and functional groups, as shown in Figure 1. Among these, 1-butanethiol and allyl sulfide are considered as nonconductive molecules, since 1-butanethiol has a long carbon chain without any double bonds, while allyl sulfide has two double bonds that are not conjugated. In contrast, both thiophenol and 4-mercaptopyridine have conjugated double bonds, which make them conductive in special circumstances.
Thiolation took place by sonicating a chosen thiol molecule solution with a liquid Ga drop in the bulk form ( Figure 1a).
As the sonication progressed, the color of the mixture turned into dark gray that indicated the formation of suspended functionalized Ga micro-and nanoparticles.

Morphological and Elemental Characterizations
The size and distribution of the spherical thiolated Ga nano-and microparticles were investigated using SEM, as shown in Figure 1f. The average diameters of 1-butanethiol-, allyl sulfide-, thiophenol-, and 4-mercaptopyridine-functionalized Ga particles were found to be 670, 710, 685, and 220 nm, respectively (Figure 2a-d). These four synthesized thiolated Ga particles presented a positively skewed trend in histograms, which is characteristic of log-normal distributions, common in emulsion and particle systems. [28] To further characterize the synthesized thiolated Ga particles, an elemental mapping using EDX equipped with SEM was performed to check the interaction between liquid Ga and thiol molecules or a reference molecule allyl sulfide. For all types of Ga particles, regions showing the Ga signals also displayed the signals of elemental oxygen and sulfur (S) as presented in Figure S2, Supporting Information. This is likely due to the thiolate ions binding to Ga through open spaces or defects on the metal surface that are not occupied by hydroxide ions or oxide layers. [29] However, the SEM-EDX was not of sufficient resolution to differentiate between the intensities of elemental S, O, and Ga in different samples.
Next, TEM (equipped with EDX) was also performed on the thiolated Ga particles. In the control sample, a large amount of solid and nonspherical flakes was observed to form during the sonication between Ga and water ( Figure 2e). Such flakes are known to be gallium hydroxide. [30] When the bulk Ga droplet disintegrates into micro-/nanosize particles via sonication, oxide layers start to grow on the surface of the particles and become thicker over time. The thick oxide layers then break into small pieces and detach from the Ga surface. This process produces oxide nanoflakes which are rod shaped (as observed in Figure 2e). However, in the presence of suitable stabilizing agents, such thick oxide layer formation can be largely prevented or reduced. [30] In contrast to the control system without stabilizers, spherical liquid Ga particles were obtained when Ga was sonicated with 4-mercaptopyridine molecules ( Figure 2f and Figure S3, Supporting Information). This evidenced that the thiol molecules stabilized the liquid Ga particles during sonication and reduced the possibility of oxide formation. Similar observations have been reported previously for a variety of surfactants. [26,31,32] The TEM-EDX elemental mapping of the synthesized Ga particles is shown in Figure 3. Among all the thiolated Ga particles, 4-mercaptopyridine Ga composites displayed the most intense signal for elemental S than the rest, indicating that 4-mercaptopyridine has stronger affinity to liquid Ga than the other three thiolated molecules. This aspect is further discussed in later sections. Such a trend was also observed for the majority of the images taken. Furthermore, the allyl sulfide samples had relatively weak S signals which might be due to the formation of unstable physisorbed molecules onto the Ga particles. Here we discuss the characteristics of 4-mercaptopyridine Ga sample in detail ( Figure 4d) in comparison with the other samples. Several characteristic peaks from 4-mercaptopyridine were detected in the 4-mercaptopyridine Ga sample. For example, there are two bands at %750 cm À1 in the FTIR fingerprint region which can be assigned to C─H bending, appearing specifically in a pyridine structure. [33] Additionally, a clear band at 1214 cm À1 indicates a C─N stretching mode. [33] The bands at 1480 and 1620 cm À1 are attributed to C═N and C═C stretching on the pyridine ring, [33] respectively. Importantly, there is a bulged region between 2500 and 3000 cm À1 which only appears in 4-mercaptopyridine precursor spectrum. Focusing on around 2500 cm À1 region, there is a small peak located near 2600 cm À1 in 4-mercaptopyridine spectrum which belongs to S─H stretching mode. [34] In comparison between the precursor and 4-mercaptopyridine Ga sample spectrum, the peak at 2600 cm À1 disappears, which possibly indicates the gallium-sulfur bond formation. This agrees well with the TEM-EDX results that S had the strongest presence on the surface. In contrast, the FTIR spectrum of allyl sulfide Ga sample lacks S─H bond (Figure 4b), which is also in agreement with the TEM-EDX results, where the S intensity on the surface is very low. In addition, 1-butanethiol and thiophenol Ga samples also show the absence of the same 2600 cm À1 peak as observed for the 4-mercaptopyridine Ga samples (Figure 4a,c).

Electrical Conductivity of Thiolated Ga Particles and References
To demonstrate that the inclusion of the selected thiol molecules during Ga sonication has a potential to increase the conductivity of the resulting Ga particles, electrical conductivities of the samples were assessed. For these assessments, silver-palladium electrodes were used for measuring the resistance of the thiolated Ga samples and then the resistance values were converted to conductivity.
Dispersions containing thiolated Ga samples were pipetted directly onto the microelectrodes and dried in a vacuum oven set at 80°C. The resistance and conductivity for Ga samples with no thiolation, all thiolated Ga samples, and their precursor standards are presented in Table 1. The comparison of conductivity with other metals (pure Ga, copper, bismuth, and indium) is also presented in Table S1, Supporting Information. By comparing the resistance of all the thiolated Ga samples, 4-mercaptopyridine Ga sample had the lowest resistance with an average value of 1.9 MΩ, which was followed by a resistance value of 13.7 MΩ for the thiophenol Ga sample. The resistance of 1-butanethiol and allyl sulfide Ga samples overload the measurement system (>40 MΩ), so they were at least 21 times higher than the 4-mercaptopyridine Ga samples and 2 times larger than the thiophenol Ga sample. Furthermore, the samples of Ga nano-and microparticles obtained with no thiolation and organosulfur standards all resulted in overloading. For nonthiolated Ga sample, the presence of thick oxide layers around the Ga particles together with the formation of the Ga hydroxide flakes resulted in high www.advancedsciencenews.com www.advintellsyst.com resistance. For three thiol molecule standards and a reference molecule allyl sulfide, the high resistance also implies that the flow of electrons was restricted in the absence of liquid Ga particles. The obtained resistance values for different samples were then converted into electrical resistivity (ρ in Ωm) using [35] where R is the measured resistance of thiolated Ga samples in Ω, A is the area of the thiolated Ga samples in m 2 (thickness of the samples was assessed using cross-sectional SEM), and l is the length of the electrodes in m. The relationship between electrical resistivity (ρ) and conductivity (σ) is expressed as σ ¼ 1 ρ [36] and its unit is S m À1 . Using the equations, the values for four thiolated Ga samples are obtained as 35.1 Â 10 À3 , 4.9 Â 10 À3 , 2.2 Â 10 À3 , <1.6 Â 10 À3 , and <1.6 Â 10 À3 S m À1 for 4-mercaptopyridine, www.advancedsciencenews.com www.advintellsyst.com thiophenol, 1-butanethiol, and allyl sulfide samples, respectively. The electrical conductivity differences among the four thiolated Ga samples are attributed to the variation of chemical functional groups of thiol molecules and a reference molecule allyl sulfide. Due to the resonance of delocalized electrons, molecules produce electron flows and generate the conductivity. [37] As a result, this gives an explanation that 4-mercaptopyridine and thiophenol Ga particles which have conjugated double bonds and resonance structures give lower electrical resistances (i.e., higher conductivity) while in 1-butanethiol and allyl sulfide, Ga particles do not have conjugated double bonds and give higher resistances (i.e., lower conductivity).

XPS and XRD Characterizations
As the best conductivity obtained for 4-mercaptopyridine Ga sample, we characterized this sample by XPS to obtain more compositional and binding information of the synthesized materials. As expected, the XPS survey spectra for the 4-mercaptopyridine Ga sample detected the compositional peaks including S 2p that confirmed the thiolation of the particles ( Figure S4, Supporting Information). From the XPS core-level spectrum of Ga 3d of the 4-mercaptopyridine Ga sample ( Figure 5), there are two peaks located at binding energies of 18.2 and 20.2 eV. The peak with the binding energy of 18.2 eV is generally assigned to metallic gallium (Ga 0 ). [38] In comparison with Ga-only (control) sample, the binding energy of Ga 3þ in 4-mercaptopyridine Ga sample was shifted from 20.0 to 20.2 eV, which may correspond to the coordination interaction between thiol molecules and Ga [39] that further confirms the existence of the S─Ga bond formation.
We have also conducted XRD analysis of the thiolated Ga samples with 4-mercaptopyridine as the control. The pure 4-mercaptopyridine diffraction pattern agrees well with the database (COD No. 96-450-5387). For the 4-mercaptopyridine thiolated Ga samples, the peak located at %32°can be attributed to a characteristic peak of solid Ga, [40] while the other small peaks correspond to Ga-OOH species. [41] Additionally, the two peaks located in the range of 15°-17°in pure 4-mercaptopyridine also appeared in the 4-mercaptopyridine thiolated Ga samples, suggesting the successful functionalization of the Ga surface by 4-mercaptopyridine ( Figure S5, Supporting Information). www.advancedsciencenews.com www.advintellsyst.com Taken together, the observed conductivity of thiolated Ga samples can be attributed to the metal-molecule junctions, where the conjugated thiol molecules attached onto the Ga surface and provided pathways for electron flow among the Ga particles. Similar observations have been made previously with electrical conduction measurements for goldÀmolecule junctions with different types of goldÀS contacts using a variety of thiol linkers. [42,43]

Sensors Based on Thiolated Ga Samples
Next, the 4-mercaptopyridine thiolated Ga sample was tested for sensing applications since it showed the highest conductivity among all the samples. The sensors were developed using flexible PET substrates (Figure 6a). Three types of sensors were developed and tested for gas, exhalation, and flex sensing. The details of the sensors' preparations can be found in the Section 4.

Response to NO 2
The 4-mercaptopyridine Ga-based sensor was used for NO 2 sensing and compared with those prepared from only liquid Ga and 4-mercaptopyridine (Figure 6a). The electrical resistance changes of the sensors exposed to 9 ppm NO 2 (balanced with air) and air were monitored. The experimental gases were moistened before introducing into the gas chamber, to avoid the overloading. For the 4-mercaptopyridine Ga-based sensor, the response time to NO 2 was less than 8 min and the recovery time was less than 7 min, while there was no resistance change from the control   Figure 6a). The mechanism of NO 2 sensing is likely based on the physisorption of NO 2 gas onto the surface of 4-mercaptopyridine Ga particles as the sensor operates at low temperature. [44,45] We hypothesized that there are noncovalent interactions between 4-mercaptopyridine and NO 2 gas molecules when adsorbed onto the surface of thiolated Ga. This is further corroborated by the photoluminescence experiments of the thiolated Ga samples in the presence or absence of NO 2 gas. While the 4-mercaptopyridine Ga sample in air exhibited an emission peak at 554 nm, this peak was shifted to 589 nm in presence of NO 2 gas ( Figure S6, Supporting Information). However, we note that a detailed mechanistic investigation will be required for the better understanding of the NO 2 sensing of such Ga systems and will be explored in our future works.

Response to Exhalation
For the tests of exhalation sensing, exhaled air from an individual was directly passed onto the sensors made of PET films covered with 4-mercaptopyridine Ga particles. After each exhalation, the observed resistance of the sensors decreased dramatically, down several folds in a few seconds, which can be attributed to the adsorption of water molecules (humidity from exhalation) onto the highly porous 4-mercaptopyridine Ga particles (Figure 6b). Note that the original resistance of the samples was recovered within 20 s after exhalation (within 90% of the baseline value). Control groups of sensors based on only liquid Ga particles and 4-mercaptopyridine were also tested for exhalation sensing. The resistances always remained overloaded after each exhalation and never showed any detectable change.

Response to Flex
For flex sensing, the 4-mercaptopyridine Ga-based PET sensor was anchored on a PU foam and squeezed until the interior angle reached %60° (Figure 6c), while the electrical resistance was continuously monitored. This process was repeated multiple times to confirm the reproducibility of the data, and after bending, the electrical resistance increased, which is attributed to the increase of distance among the 4-mercaptopyridine covered Ga particles. The solutions of Ga particles-only and 4-mercaptopyridine-only samples were also separately drop cast onto PET films and the same process was performed. The measurements showed overloading for both the control groups. The reason of overloading for only Ga might be due to formation of thick Ga oxide layers on the surface of Ga particles, causing the loss of conductivity. In the case of 4-mercaptopyridine only, it is known that unless the conjugated molecules are polymerized, no electron flow can be observed.

Conclusion
In summary, we demonstrated the possibility of coating thiol molecules on the surface of liquid Ga particles at nano-and microscale. We found that conjugation was essential to introduce the conductivity to micro-and nanoparticles mixes. Three different thiol molecules, and a reference molecule allyl sulfide, were chosen: two with conjugations and two without. The highest conductivity was obtained for the 4-mercaptopyridine Ga mix. When 4-mercaptopyridine was used, thiolation took place more efficiently on the surface of Ga to suppress the Ga oxide layer formation and provided the conductivity through conjugating pathways at the same time, without fusing the particles together. In contrast, thiol molecules without conjugation showed high resistivity. Devices were successfully developed to show the functionality of the thiolated Ga particles with 4-mercaptopyridine for gas, exhalation, and flex sensing and repeatability was observed. This work provides insights into the efficient combination of thiol molecules with Ga-based micro-and nanoparticles for developing conductive mixes that do not require any sintering process for gaining conductivity. It also presents an idea of selecting different organic molecules with a variety of functionalities that could combine or interact with other transition metals to develop functional systems.
Synthesis of Thiolated Molecules/Ga Particles: Micro-/nanoparticles of Ga, with thiolated coatings, were prepared by the sonication of Ga bulk in liquid state. Liquid Ga was prepared by heating a sealed glass vial on a hot plate at 40°C. Then, the molten Ga was stored in a capped glass vial and put it into an oven at 40°C for further use.
In a glass vial, 5 mL of dimethyl sulfoxide (DMSO) was poured into which 10 mL of aqueous NaOH solution was added to make a final concentration of %0.07 M NaOH. The solvent, DMSO, was chosen to provide higher solubility for all the thiolated molecules. The purpose of NaOH www.advancedsciencenews.com www.advintellsyst.com solution is to remove the Ga surface oxide layer and provide a basic environment for the thiolated molecules. An ultrasonic bath sonicator (FXP10DH, Unisonics Australia Inc.) was used for the synthesis of the thiolated Ga nano-and microparticles. First, a thiol molecule (1-butanethiol, thiophenol, or 4-mercaptopyridine) or an organosulfur compound (allyl sulfide) was added to the prepared solvent solution in the glass vial and mixed thoroughly using a vortex mixer for 30 s. After that, the precursor solution was placed in an ultrasonic bath sonicator (preheated to 50°C) for 10 min for further processing. Then, 60 μL of liquid Ga was pipetted to the preheated precursor solution to avoid Ga becoming solid during the sonication process. Finally, the obtained mixture was placed in an ultrasonic bath sonicator and the sonication time was set to 25 min at 50°C. During sonication, the solution turned into dark gray color due to acoustic stimulation, indicating the formation of Ga nano-/microparticles.
Characterization: SEM and TEM Imaging and Elemental Mapping: The size and elemental distribution of thiolated Ga particles were characterized by scanning electron microscopy (SEM, JEOL JSM-IT 500 HR) equipped with energy-dispersive X-Ray spectroscopy (EDX) and a high-resolution transmission electron microscopy (HRTEM, FEI Tecnai G2 20) unit also equipped with EDX (Bruker Quantax). These were used for the analysis of morphology, size, and elemental distribution of the thiolated Ga particles. Samples for SEM were prepared by drop casting the thiolated Ga particles onto a Si/SiO 2 wafer, while samples for TEM imaging were prepared by depositing the particle suspensions onto carbon-coated copper grids (400 mesh). All the samples for SEM and TEM imaging were washed three  The X-Ray photoelectron spectroscopy (XPS, ESCALAB250Xi, Thermo Scientific) analysis was performed using a monochromated Al Kα X-Ray source (1486.68 eV) at the power of 120 W (13.8 kV and 8.7 mA). The X-Ray spot size was 500 μm and the photoelectron take-off angle was 90°. The incident radiation was monochromatic Al Kα X-Ray (1486.68 eV) at 120 W (13.8 kV and 8.7 mA). All data were processed using Advantage Software and the energy calibration was referenced to the C 1s peak at 284.8 eV.
The crystalline phases of 4-mercaptopyridine thiolated sample and pure 4-mercaptopyridine were characterized by XRD (MPD Xpert Multipurpose). All measurements were carried out using Cu Kα X-Ray radiation with a wavelength of 1.54 Å at room temperature.
Electrical Conductivity Measurements: To measure the conductivity of synthesized thiolated Ga particles, substrates with printed interdigital electrodes (93.8 mm 2 surface area, JT-H083, purchased from Berun) which were composed of eight printed silver-palladium (Ag-Pd) fingers of parallel sides on an alumina substrate with 0.2 mm spacings (patterns are shown in Figure S1, Supporting Information) were used. The conductivity measurements were carried out by drop casting 10 μL of the thiolated Ga particle dispersions onto the center of Ag-Pd electrodes. Then, the samples were dried in a vacuum oven (at 80°C for 1.5 h) for solvent evaporation before measuring the electrical resistance by a digital multimeter. Each experiment was repeated three times for all the different types of thiolated Ga particles, standards (1-butanethiol, allyl sulfide, thiophenol, and 4-mercaptopyridine), and Ga particles (without thiolation) to obtain the average results of electrical resistances. The resistance values were used for resistivity (ρ) calculation by considering the total width of the electrodes and finally converting them into conductivity (σ).
Sensing Measurements: To further demonstrate the capability of proposed thiolated Ga particles, the solution of 4-mercaptopyridine Ga nanoparticles was drop cast (%150 μL) on a scissored polyethylene (PET) film (30 mm Â 5 mm). Then the PET film was dried in a vacuum oven (at 80°C for 1.5 h) for solvent evaporation. Next, a 50 mg silver paste was added onto two ends of the PET film and covered by 2 cm copper-conductive metallic tapes for further sensing measurements.
Sensing Measurements: Development of Sensors Based on 4-Mercaptopyridine Ga Composites: For sensing applications, the PET film containing 4-mercaptopyridine Ga composites was used for gas, exhalation, and flex sensing.
For gas sensing, the device was placed in a LINKAM gas chamber (LINKAM HFS600E-PB4). The electrical resistance was recorded using a lab view-controlled digital multimeter (Siglent SDM3065X, Siglent Tech-nologies). For NO 2 sensing, two mass flow controllers were set for two cycles to monitor the flow rate (100 sccm) of two gas inlets alternatively, which were filled with air and 9 ppm of NO 2 (balanced with air). Before the gases entered directly into the LINKAM gas chamber, they were passed through a tube containing DI water for humidification to near room humidity to avoid overloading in the multimeter that might affect the judgment of sensing measurements. Tests were also conducted on two control groups (Ga only and 4-mercaptopyridine only) to make a comparison with 4-mercaptopyridine Ga particles.
For exhalation sensing, the device was directly exposed to exhalation from individual and the data acquisition took place by recording the electrical resistance using the lab view-controlled digital multimeter as mentioned above.
For flex sensing, the PET device was attached onto a flexible polyurethane (PU) foam and two sides of the copper conductive tapes on PET film were gripped by grippers connected to the lab view-controlled digital multimeter. One side of the PE foam was fixed while the other side was gradually squeezed by a step motor of a syringe pump (Chemyx Fusion 4000, Chemyx Inc.) with a velocity of 0.25 mm s À1 until the PU foam was bent at an angle of %60°. The processes were repeated for multiple cycles to measure the resistance difference and assess the repeatability. However, only two cycles are presented in the article for simplicity.
Photoluminescence Measurements: For the photoluminescence measurements, sample preparation was the same as described in the gassensing experiments in the section of Sensing Measurements. The LINKAM gas chamber was placed inside a Raman/photoluminescence system (inVia, Qontor) microscope. The locations of the 4-mercaptopyridine and 4-mercaptopyridine thiolated Ga samples were focused onto. The photoluminescence spectra were then obtained by applying a laser source (355 nm) to excite 4-mercaptopyridine and 4-mercaptopyridine thiolated Ga samples separately and their emission spectra were compared.