# Positive to negative photoconductance switching in plasmonic gold nanoparticle networks

Louis Merle, A. Mlayah, T. Dammak, Simon Tricard, J. Grisolia

► To cite this version:

Louis Merle, A. Mlayah, T. Dammak, Simon Tricard, J. Grisolia. Positive to negative photoconductance switching in plasmonic gold nanoparticle networks. Materials Research Bulletin, 2026, 193, pp.113705. 10.1016/j.materresbull.2025.113705. hal-05339531

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## Research Papers

# Positive to negative photoconductance switching in plasmonic gold nanoparticle networks

L. Merle<sup>a</sup>, A. Mlayah<sup>b, c</sup>, T. Dammak<sup>c</sup>, S. Tricard<sup>a</sup>, J. Grisolia<sup>a, *, d</sup>

<figure><img src="image_2.png" /></figure>

### ARTICLE INFO

**Keywords:**

Plasmoelectronic

Positive and negative photoconductance

Gold nanoparticles self

Assembly

Impedance spectroscopy

### ABSTRACT

The elaboration of versatile materials in which electrical conduction is tuned by light irradiation is of paramount relevance to such diverse applications as photodetectors, photodiodes, solar cells or light sensors. Although positive photoconductance is increasingly common, systems with negative photoconductance are scarcely reported. In this work, a switching from positive to negative photoconductance is observed upon simple annealing of well-organized networks of gold nanoparticles stabilized by dodecanethiols. The photoconductance properties are investigated experimentally using impedance spectroscopy. The measured Nyquist plots and resonance curves of the impedance are analyzed in terms of equivalent electrical circuits consisting in parallel resistance, capacitance and photoconductance. The positive and negative photo-current conversion efficiencies of the nanoparticles network are estimated $k_{PPC} = 389 \pm 40 \text{ nS.W}^{-1}\text{cm}^2$ and $k_{NPC} = -241 \pm 40 \text{ nS.W}^{-1}\text{cm}^2$, respectively. With the aid of density functional theory calculations, the origin of the photoconductance is discussed, at the nanoscale level, in terms of changes of molecular conformation. Such molecular effects assist charge carrier tunneling between first neighbor nanoparticles, and favor the formation of traps introduced by the annealing of the sample. The present work contributes to the understanding of plasmo-electronic properties of hybrid molecule-nanoparticle self-assembled nano-structures.

## 1. Introduction

The link between charge transport and surface plasmons in metal nanostructures is driving the burgeoning field of plasmo-electronics, opening the door to a new class of light-responsive nano-devices [1–5]. While charging metal nanoparticles (NPs) can impact their plasmonic response, plasmonic excitation of nanoparticle assemblies has been demonstrated to influence macroscopic charge fluxes [6–8]. Interestingly, these interactions have resulted in both positive and negative photoconductances (PPC and NPC respectively), whereby the conductance either rises or falls in response to an optical irradiation tuned to the surface plasmon resonance of the NPs [9,10].

The phenomenon of positive photoconductance (PPC) is well-established in scientific literature. It occurs when a material's conductivity increases due to a higher concentration of charge carriers within it. This effect is directly linked to the process of photo-generation, where light creates additional charge carriers in the material. In the case of metal NP networks, PPC can be significantly enhanced at specific

wavelengths corresponding to plasmonic resonances [11]. On the other hand, negative photoconductance (NPC), i.e. a decrease in conductivity upon optical excitation, has been observed in various material structures including bulk materials, thin films, and low-dimensional nano-structures [12–19]. While considerable advancements have been made in understanding NPC, there remains a gap in comprehension of how local opto-electronic properties at the molecular and nanoscopic scales determine the macroscopic characteristics of NPC, especially its spectral dependence [20].

Various interpretations have been proposed for NPC. Nakanishi and colleagues suggested that negatively charged ligands act as traps, reducing free carrier density [6]. However, this explanation was challenged by observations of positive photoconductance with charged ligands. Alternative explanations for NPC include charge trapping due to defects, surface adsorbents, and doping. Additionally, the adsorption and desorption of gas molecules have been linked to NPC [21,22]. Changes in charge carrier concentrations or mobility have also been proposed as potential causes for decreased conductivity [10].

<footnote><sup>a</sup> LPCNO, Université de Toulouse, INSA, CNRS, UPS, 135 Avenue de Rangueil, Toulouse 31077, France</footnote>

<footnote><sup>b</sup> LAAS, Université de Toulouse CNRS, UPS, Toulouse, France</footnote>

<footnote><sup>c</sup> University of Sfax, BP 1169, 3029 Sfax, Tunisie</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: jeremie.grisolia@insa-toulouse.fr (J. Grisolia).</footnote>
Understanding these mechanisms (PPC and NPC) is crucial for advancing technology in various fields, including photoelectric detection, gas sensing, memory devices, and optoelectronic switching. They are in particular of significant interest in the development of advanced light-responsive nanodevices [14–17],[23,24].

The observation of PPC and NPC in self-assembled gold NP stabilized by dodecanethiol molecules is reported in this work. After the sample is annealed, a transition from PPC to NPC is seen. The spectrum responses of PPC and NPC are examined using impedance spectroscopy techniques. Both macroscopic and nanoscopic equivalent electrical circuits are considered to analyze the experimental results. Estimates are made of the PPC and NPC efficiencies. It is discussed how light is transformed into charge carriers and how current flows via the network of nanoparticles. Calculations using density functional theory are used to provide a general approach for both PPC and NPC. The distinctive characteristic of this work is that the same system being investigated can display both PPC and NPC.

## 2. Experimental details

The sample preparation is detailed in a previous paper [11]. Briefly, 7 nm gold NPs functionalized with dodecanethiol organic ligands were synthesized with the protocol described by Zheng et al. [25] (Figure S1 in ESI). On another hand, interdigitated gold electrodes were patterned on 125 µm polyimide (Upilex) films using laser lithography and a 50 nm gold Au layer was deposited over the entire surface of the substrate by IBS-E ion beam sputtering. After lift-off, a 1 mm² device consisting in 50 pairs of interdigitated electrodes 5 µm wide and 5 µm interdigitated distance was obtained and connected by external gold electrodes [26]. Finally, the gold NPs were deposited between the interdigitated electrodes by evaporation of an ethylene glycol droplet to assemble NPs on the interdigitated electrodes [26]. This assembly approach allows depositing a uniform monolayer of self-assembled nanoparticles, generating a homogenous film on a large area (1 mm²).

Electric characteristics were measured using a Janis ST-500-1 cryogenic probe station coupled to an impedance spectroscopy measurements set-up, a Keysight Impedance Analyzer EA4990A, operating in the 20Hz-5 MHz frequency range with a 500 mV AC voltage. The cryogenic station is equipped with an optical fiber that is positioned above the sample in order to irradiate the entire area of interest. This optical fiber is connected to a set-up of various laser diodes with adjustable intensity. In all the here presented experiments, the excitation wavelength of 532 nm was chosen close to the plasmonic resonance of the NP network observed around 540 nm (Fig. 1c in ESI). So, both PPC and NPC experienced in this work are amplified by the surface plasmon properties.

## 3. Impedance spectroscopy results and analysis

### 3.1. Without optical excitation

Fig. 1 presents room-temperature impedance spectroscopy measurements in the form of Nyquist's plots (-Z'' versus Z') and -Z'' resonance spectra. The measurements were performed without optical illumination on the as-prepared sample before and after annealing at a temperature of 345 K and a duration of 15min. All Nyquist plots have nearly a semicircle shape which is a characteristic feature of an equivalent electrical circuit consisting in macroscopic parallel resistance R and capacitance C (Fig. 1). The -Z'' curves show a resonant peak located at $f_0 = \frac{1}{2\pi RC}$.

The Nyquist's plots and -Z'' resonance curves were calculated and fitted to the experimental data using a single (R, C) parallel circuit (Fig. 1a, b) to determine R and C. We obtain R = 2.5MΩ (before annealing) and R = 0.75MΩ (after annealing) and C = 15.2pF (before annealing) and C = 14.1pF (after annealing) (see Table 1). The

<figure><img src="image_3.png" /><figcaption>Fig. 1. (a) Comparison between measured (dots) and calculated (full and dash-dotted lines) impedance Nyquist plots before and after annealing of the sample (without optical excitation) and (b) corresponding spectra of -Z''(ω). The dash-dotted lines in (a) corresponds to a perfect semi-circle. The black full lines in (a) and (b) show the results of calculations performed for a perfectly order nanoparticle network characterized by a single relaxation time. The others full lines in (a) and (b) are corresponding to a simulated disordered nanoparticle network with a gaussian distribution of relaxation times, used to model the Nyquist plots and resonance curves.</figcaption></figure>

<caption>**Table 1**</caption>

Electrical equivalent circuit parameters (R and C) and characteristics ($\bar{\tau}(0)$, $\Delta\bar{\tau}(0)$) of the relaxation time distribution extracted from the fitting of the calculated impedance (Eq.1) to the experimental data (Fig. 1).

<table><thead><tr><th></th><th>R (M)</th><th>C (pF)</th><th>τ̄(0)(s)</th><th>Δτ̄(0) (s)</th></tr></thead><tbody><tr><td>Before annealing</td><td>2.5</td><td>15.2</td><td>3.8 × 10<sup>-5</sup></td><td>1.6 × 10<sup>-5</sup></td></tr><tr><td>After annealing</td><td>0.75</td><td>14.1</td><td>1.1 × 10<sup>-5</sup></td><td>5.4 × 10<sup>-6</sup></td></tr></tbody></table>
agreement between calculations and measurements is satisfactory. However, one can notice that the measured Nyquist's plots shown in Fig. 1a are not perfect semi-circles; they are depressed, i.e. the ratio between the maximum values of the imaginary and real parts of the impedance is lower than 0.5, $-Z'_{max}/Z_{max} < 0.5$; for a perfect semi-circle $-Z'_{max}/Z_{max} = 0.5$. Correspondingly, the line shape of the $-Z''$ resonance peak is not accurately described by a Lorentzian function, especially around the resonance frequency (Fig. 1b). This indicates that the equivalent circuit is formed by a distribution of parallel resistance and capacitance circuits rather than by a single of such a circuit (insert of Fig. 1b). The distribution of circuits is generated by the disorder due to size and position fluctuations of the nanoparticles and also by point defects (nanoparticle vacancies) and dislocations present in the network [27]. Indeed, the interparticle resistance $R_{ij}$ and capacitance $C_{ij}$ forming a nano-junction may fluctuate from one nano-junction to another. Therefore, the local relaxation time defined as $\tau_{ij} = R_{ij}C_{ij}$ is a local characteristic of the dynamical response of a nano-junction and may fluctuate spatially. Assuming a continuous distribution of relaxation times, the overall impedance of the network reads:

$$Z(\omega) = R_s + R \cdot \int_{0}^{\infty} \frac{g(\tau)}{1 + j\omega\tau} d\tau \quad (1)$$

Where $g(\tau)$ is the normalized relaxation time distribution $\int_{0}^{\infty} g(\tau) d\tau =$

1; R is the macroscopic resistance of the nanoparticle network and Rs is a series contact resistance. The latter is determined from the impedance measured at high frequency; here we obtain $R_s \sim 100\Omega$.

In a perfectly ordered network, where all the interconnected nano-junctions are identical, the distribution function is a delta function centered on a characteristic nanojunction relaxation time $\tau_{ij} = R_{ij}C_{ij}$ (all interparticle resistance $R_{ij}$ and capacitance $C_{ij}$ are identical). In that case, the macroscopic resistance and capacitor of the nanoparticle network are, respectively, $R_{ij}N/M$ and $C = C_{ij}M/N$ where N and M are the numbers of series and parallel nano-junctions between the contact electrodes, respectively. Hence, the macroscopic and microscopic relaxation times are identical, i.e., $RC = R_{ij}C_{ij}$. The Nyquist's plot of such a perfect network is a semi-circle and its $-Z''$ resonance curve has a Lorentzian lineshape centered at the resonance frequency $f_0 = \frac{1}{2\pi\tau_{ij}} = \frac{1}{2\pi RC}$, as shown in Fig. 1. It is worth to underline that while Nyquist's plots reveal the macroscopic resistance R and capacitance C, the $-Z''$ resonance curve is peaked at the frequency $f_0$ determined by the nano-scopic resistance $R_{ij}$ and capacitance $C_{ij}$. Hence, Nyquist plots and $-Z''$ resonance curve bring complementary information at the macroscopic and nanoscopic scales.

To describe our actual disordered nanoparticle network, we assume a

gaussian distribution of relaxation times $g(\tau) = \frac{1}{\Delta\tau}e^{-\frac{(\tau-\bar{\tau}(0))^2}{\Delta\tau(0)^2}}$. where $\bar{\tau}(0)$ and $\bar{\Delta\tau}(0)$ are the average relaxation time and the standard deviation (in the absence of any optical excitation) which are used to fit the measured Nyquist's plots (Fig. 1a) and the resonance curves (Fig. 1b), simultaneously. A very good agreement between calculations and measurements is obtained (Fig. 1): the depressed character of the Nyquist's plots as well as the non-Lorentzian lineshape due to inhomogeneous broadening of the resonance curves are well accounted for. The values of $\bar{\tau}(0)$ and $\bar{\Delta\tau}(0)$ obtained before and after annealing of the sample are quoted in Table 1.

One can notice that, after annealing, $\bar{\tau}(0)$ and $\bar{\Delta\tau}(0)$ are reduced by nearly a factor 3.6 (Table 1). due to the decrease of the macroscopic resistance ($R = 2.5M\Omega$ to $R = 0.75M\Omega$) whereas the macroscopic capacitance is almost constant ($C = 15.2$ pF to 14.1 pF) as already discussed above. The impedance spectroscopy data analyzed using macroscopic and microscopic equivalent circuits (Fig. 1) indicate that after annealing the interparticle conductance is increased. This

observation is discussed in more details below and is directly connected to the observation of PPC to NPC switching.

## 3.2. With optical excitation

Optical excitation, before and after annealing of the sample, has been performed at laser wavelength of 532 nm close to the plasmonic resonance of the NP network (Fig. 1c in ESI), with laser intensity increasing up to 1 W/cm². At such a low irradiation intensity laser induced sample heating is ruled out, as heating becomes significant at intensities larger than 1 kW/cm² [28,29]. All Nyquist plots have a depressed semicircle shape due to the disorder effects discussed above. Two opposite behaviors are observed (Fig. 2): (i) before sample annealing, the electric resistance decreases with increasing laser intensity which is the signature of PPC. (ii) on the contrary, after annealing, the electrical resistance clearly increases with increasing laser intensity, which indicates NPC. It is remarkable that the same sample (same NPs and ligand molecules) could exhibit both PPC and NPC. To account for the effect of optical excitation, a photo-resistance $G_{photo}^1$ is added in parallel to the equivalent macroscopic circuit (Fig. 2). The resistance of this circuit is $\frac{R}{1+RG_{photo}}$ in the low frequency limit $f \ll f_0$. To account for the observed PPC and NPC, $G_{photo}$ can be either positive or negative.

By fitting the Nyquist's plots (Fig. 2) and the $-Z''$ resonance spectra (see Figure ESI 2) using Eq.1 and adjustable gaussian distributions of relaxation times and adjustable $G_{photo}$, we are able to determine the dependence of $\bar{\tau}$, $\Delta\tau$ and $G_{photo}$ on laser intensity, for both PPC and NPC. As shown in the insets of Fig. 2, we found that both PPC and NPC vary linearly with the laser intensity which confirms the absence of thermal effects due to sample heating known to be responsible for nonlinear variations [28]. We determine the slopes of these linear variations as $k_{PPC} = 389 \pm 40 nS \cdot W^{-1} \cdot cm^2$ for PPC and $k_{NPC} = -241 \pm 40 nS \cdot W^{-1} \cdot cm^2$ for NPC. These values are macroscopic photo-conversion coefficients which characterize the conversion efficiency of the incident photons into charge carriers flowing through the NP network and are discussed in more details in the next section. Fig. 3 displays the variations of the average relaxation time and its standard deviation as a function of laser intensity for both PPC and NPC.

The variation of the average relaxation time can be understood in terms of a local average nanojunction consisting in parallel $R_{ij}$, $C_{ij}$ and photo-resistance $1/\bar{g}_{ij}(I_0)$, where the interparticle photoconductance $\bar{g}_{ij}(I_0)$ depends on laser intensity (and wavelength), and can be either positive or negative. Assuming that only the interparticle resistance is affected by the size and the position fluctuations of the NPs (due to the exponential variation of the tunneling resistance with interparticle gap), the average local relaxation time and its standard deviation are, for $\bar{R}_{ij}\bar{g}_{ij}(I_0) \ll 1$, given by:

$$\bar{\tau}(I_0) \approx \bar{\tau}(0)(1 - \bar{R}_{ij}\bar{g}_{ij}(I_0)) \quad \text{and} \quad \bar{\Delta\tau}(I_0) \approx \bar{\Delta\tau}(0)(1 - 2\bar{R}_{ij}\bar{g}_{ij}(I_0)) \quad (2)$$

Where $\bar{\tau}(0)$ and $\bar{\Delta\tau}(0)$ are the average relaxation time and standard deviation without optical excitation (Figure 1 and Table 1).

$\bar{R}_{ij} = (M/N)R$ is the average resistance of a nanojunction deduced from the macroscopic resistance R and the numbers N and M of series and parallel nanojunctions, respectively (see suppl. Info). The M/N ratio is around $10^4$, which gives $\bar{R}_{ij} = 2.5 \times 10^{10} \Omega$ for PPC and $\bar{R}_{ij} = 7.5 \times 10^9 \Omega$ for NPC.

Assuming a linear variation, in Eq. 2, of the local photo-conductance with laser intensity $\bar{g}_{ij}(I_0) = k_{ij}I_0$, the data of Fig. 3 can be accounted for using the average nanoscopic photo-conversion efficiency $\bar{k}_{ij}$ as an adjustable parameter.

As can be seen in Fig. 3 the variation of both the average relaxation time and its standard deviation with laser intensity are well fitted by the linear variation of $\bar{g}_{ij}$ and we obtain: $\bar{k}_{ij} = 0.018 nS \cdot W^{-1} \cdot cm^2$ for PPC and $\bar{k}_{ij} = -0.03 nS \cdot W^{-1} \cdot cm^2$ for NPC. At the maximum laser intensity $I_0 =$
<figure><img src="image_4.png" /><figcaption>Fig. 2. Comparison between measured (dots) and calculated (full lines) Nyquist plots with optical excitation at $\lambda = 532$ nm and intensities increasing from 0 to 1W/cm$^2$ (a) before and (b) after annealing of the sample. The insets in (a) and (b) show the variation of, respectively, the positive and negative photoconductance (PPC and NPC) with excitation laser intensity at low frequency $f = 20$Hz.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 3. Average relaxation time $\bar{\tau}$ and standard deviation $\Delta\bar{\tau}$ as a function of excitation laser intensity for (a) PPC and (b) NPC. The lines show the results of a linear least square fitting to the experimental data (extracted from Figure ESI 2).</figcaption></figure>

$1$W/cm$^2$, $R_{ij}g_{ij}(I_0) \approx 0.46$ for PPC and $-0.3$ for NPC and higher order corrections to Eq. 2 do not bring significant deviation to the linear first order variations (Eq. 2). Interestingly, it is worth noting that the macroscopic over nanoscopic photoconversion efficiencies ratio is $k_{PPC}/\bar{k}_{ij} \sim 2 \times 10^4$ for PPC and $k_{NPC}/\bar{k}_{ij} \sim 0.8 \times 10^4$ for NPC which is consistent with the ratio of the numbers of parallel and serie nano-junctions ratio $M/N \sim 10^4$. The parameter $\bar{k}_{ij}$ describes the generation of PPC and NPC at the level of an average single nanojunction. Its origin lies in the local process taking place at the nanoscale and by which the absorbed photons may increase or decrease charge transfer between first neighbor nanoparticles, a point which we now address.

## 4. Positive and negative photoconductance mechanisms

Sample annealing is the reason for the observed switching from PPC to NPC (Fig. 2). We therefore postulate that the dodecanethiol molecules attached to the NP surface experience conformational and chemical changes responsible for charge carrier trapping, which affect the inter-

particle charge transfer and, in turn, the current flowing through the network. Charge carrier trapping has been proposed to explain the appearance of NPC [10]. Conductance decreases as a result of trap centers capturing the photo-generated carriers [8,30], [31,32]. Here, we assume that the photo-generated carriers are trapped when the sample is annealed. Therefore, we introduce a charge transfer scheme (Fig. 4) based (i) on the occupied (HOMO) and unoccupied (LUMO) energy levels of the dodecanethiol molecule calculated by Density Functional Theory (DFT, see ESI) and (ii) on the assumption that trap states with energy level in the HOMO-LUMO gap of the molecules appear upon sample annealing.

In the DFT calculations, we have optimized the geometry of the 1-dodecanethiol (C12H25SH) molecule based on the crystallographic data. Once the structures were optimized by DFT, the TD-DFT method was used to analyze electronic properties, such as HOMO-LUMO levels and the optical absorption spectrum. Given the dynamic interactions between the thiol groups and the nanoparticle surface, as well as the flexibility of the alkyl chains, it is likely that the molecule undergoes
<figure><img src="image_6.png" /><figcaption>Fig. 4. (a) Energy diagram for the NPC case with the trap (after annealing).</figcaption></figure>

significant conformational changes once bound to the surface. There-
fore, we have conducted a conformational study. Indeed, several distinct
conformations were optimized by DFT, notably by selectively modifying
certain angles and dihedrals, particularly on the sulfur side, a key
element for binding to the nanoparticle surface. For each conformation,
HOMO-LUMO levels and absorption spectra were calculated by TD-DFT.
Fig. 3 in ESI presents a selection of 25 conformations with their asso-
ciated electronic and optical properties. The dispersion observed in the
HOMO and LUMO energy levels for these conformations is consistent
with the hypothesis that annealing induces the formation of trap states
within the molecular band gap.

It is worth mentioning that the average separation between first
neighbor NPs is 2.8 nm (Fig. 1 in ESI). With such a gap, direct tunneling
(i.e., without the aid of molecular states) is negligible. Moreover,
because of interdiggidation of the molecules attached to the surface
neighboring NPs do interact thus providing a mean for intermolecular
charge transfer within the bridge. The molecules forming the bridge are
unlikely to be in the same conformational state. Therefore, we consider
transitions assisted by conformational changes of the molecules bridging
two NPs (Fig. 4 in ESI). Such transitions are ultimately mediated by inter
and intramolecular vibrations. Actually, the large density of conforma-
tional molecular states provides a large number of such charge transfer
channels which favors the electron tunneling and leads to positive
contribution to the photoconductance.

On the other hand, as shown in Fig. 4, trap states may appear in the
HOMO-LUMO gaps upon sample annealing. Such states may capture the
photo-excited electrons. Trapped carriers lead to charging of the mole-
cules which act as Coulomb barriers against inter-particle charge
transfer resulting in negative contribution to the photoconductance as
suggested in previous works [6]. At the same time, such trap levels may
provide additional channels for interparticle charge transfer in the
absence of optical excitation thus leading to a decrease in the overall
resistance (as observed in Fig. 1, after sample annealing).

From the analysis of the data of Fig. 3, we obtain the positive photo-
conversion efficiency $\bar{k}_{ij,\text{PPC}} = 0.018 \text{ nS.W}^{-1}\text{cm}^2$, before annealing of

the sample. After annealing, the average photoconductance is negative
and the average photo-conversion efficiency is $\bar{k}_{ij,\text{NPC}} = -0.03 \text{ nS.W}^{-1}\text{cm}^2$. This simplified approach gives a rough estimate of the
positive and negative photoconversion efficiencies.

Determining the exact nature of the trap states is challenging. They
may result from annealing-induced changes in the chemical bond be-
tween the molecules and the NPs, from intermolecular bonding within
the molecular bridges, or from any other chemical reaction involving
water molecules or environmental pollutants.

Finally, several recent studies have reported coexisting positive and
negative photoconductance effects (PPC and NPC) in nanoscale systems.
For example, Yang et al. observed both PPC and NPC in individual InAs
nanowires NPC dominating at low optical intensity due to hot-electron
trapping in surface oxide layers, and at higher intensities, PPC pre-
vailed [33]. It is worth noting that the electrical conversion coefficient
k$_{NPC}$ in this case is approximately -12 nS.W$^{-1}$.cm$^2$, which is about 20
times lower (in absolute value) than the one found in our Au nano-
particle network. Similarly, networks of Ag nanowires display NPC via
bolometric effects caused by local heating at plasmonic hotspots [34]. In
this case, k$_{NPC}$ conversion values are even lower than previously re-
ported, about $-10^{-3} \text{ nS.W}^{-1}\text{cm}^2$. Interestingly, Basyooni et al. [35]
found that annealing causes the emergence of NPC after PPC in MoO$_3$/Ir
thin films. Indeed, annealing enhances NPC at zero bias due to higher Ir
diffusion and phonon scattering, whereas PPC is found in unannealed
samples. Moreover, Wang et al. [36] reported gate-tunable switching
between PPC and NPC in 2D van der Waals heterostructures (ReS$_2$/h
BN/MoS$_2$), which is triggered by charge transfer between the floating
gate and the conducting channel. Finally, graphene quantum-dot de-
vices formed on SiO$_2$/Au electrodes demonstrate NPC with hysteresis,
which is highly dependent on ambient humidity conditions [21].

In this perspective, our observation of both PPC and NPC in the same
plasmonic nanoparticle network under comparable lighting conditions
highlights the complex interplay of light-matter interaction, carrier
dynamics, and interparticle coupling in this architecture. While the
magnitudes and physical origins may differ, our system has several
similarities with previously reported ones, including a competition between light-induced carrier excitation and trapping and the role of plasmon-mediated heating or scattering, and sensitivity to structural or environmental parameters (for example, annealing, humidity). This shows that the interaction of light matter, carrier dynamics, and interparticle coupling in self-assembled plasmonic nanoparticle networks can result in complex photoconductive behavior that warrants further investigation. For example, while the phenomena has been reproduced at various annealing temperatures and durations, a systematic investigation of the relationship between annealing parameters and the emergence of NPC is interesting. Such an investigation is underway and will be reported in a future publication.

## 5. Conclusion

In summary, we have reported the observation of an annealing-induced transition from PPC to NPC in a network of self-assembled gold NPs coated with dodecanethiols. We estimated the macroscopic photoconductance efficiencies and correlated them with the local photoconductance of the nano-junctions created by the first neighbor nanoparticles interconnected by dodecanethiols molecules. The charge transfer dynamics between the NPs were analyzed through DFT calculations, and considered the possible emergence of trap states following the annealing process. We propose that the observed NPC may stems from a competition between (i) the charge transfer facilitated by intermolecular interactions and conformational alterations of the molecules forming the inter-particle connections, and (ii) the trapping of photo-generated charge carriers that arise from the annealing treatment of the sample. This study provides insights into the intricate optoelectronic characteristics of self-assembled NP networks, which could be utilized in resistive and capacitive sensing devices.

## CRediT authorship contribution statement

L. Merle: Data curation. A. Mlahay: Writing – review & editing, Writing – original draft, Validation, Methodology, Funding acquisition, Formal analysis, Conceptualization. T. Dammak: Data curation. S. Tricard: Writing – review & editing. J. Grisolia: Writing – review & editing, Writing – original draft, Visualization, Validation, Software, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

This study has been supported through the ANR-23-CE09-0006-01 (DINAPO) and the EUR grant NanoX No. ANR-17-EURE-0009 in the framework of the Programme des Investissements d'Avenir (NAMAS).

## Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.materresbull.2025.113705.

## Data availability

Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.

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