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. 2024 Apr 18;16(17):22421–22432. doi: 10.1021/acsami.4c02102

Lanthanum(III)hydroxide Nanoparticles and Polyethyleneimine-Functionalized Graphene Quantum Dot Nanocomposites in Photosensitive Silicon Heterojunctions

Aslıhan Anter †, Elif Orhan †,*, Murat Ulusoy †, Barış Polat ‡, Mustafa Yıldız §, Arun Kumar ∥, Antonio Di Bartolomeo ∥,*, Enver Faella ⊥, Maurizio Passacantando ⊥, Jinshun Bi #
PMCID: PMC11071049  PMID: 38634639

Abstract

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Lanthanides are largely used in optoelectronics as dopants to enhance the physical and optical properties of semiconducting devices. In this study, lanthanum(III)hydroxide nanoparticles (La(OH)3NPs) are used as a dopant of polyethylenimine (PEI)-functionalized nitrogen (N)-doped graphene quantum dots (PEI-NGQDs). The La(OH)3NPs-dopedPEI-NGQDs nanocomposites are prepared from La(NO)3 in a single step by a green novel method and are characterized by Fourier-transform infrared spectroscopy (FT-IR), ultraviolet–visible spectroscopy (UV–vis), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). Deposited over an n-type Si wafer, the La(OH)3NPs-dopedPEI-NGQDs nanocomposites form Schottky diodes. The I–V characteristics and the photoresponse of the diodes are investigated as a function of the illumination intensity in the range 0–110 mW cm–2 and at room temperature. It is found that the rectification ratio and ideality factor of the diode decrease, while the Schottky barrier and series resistance increase with the enhancing illuminations. As a photodetector, the La(OH)3NPs-dopedPEI-NGQDs/n-Si heterojunction exhibits an appreciable responsivity of 3.9 × 10–3 AW–1 under 22 mW cm–2 at −0.3 V bias and a maximum detectivity of 8.7 × 108 Jones under 22 mW cm–2 at −0.5 V. This study introduces the green synthesis and presents the structural, electrical, and optoelectronic properties of La(OH)3NPs-dopedPEI-NGQDs, demonstrating that these nanocomposites can be promising for optoelectronic applications.

Keywords: rare earth elements, lanthanum(III) hydroxide doping, graphene quantum dots, green method, nanocomposite diode, photosensitivity

1. Introduction

Technological progress largely depends on the development of semiconductor-based devices. The Schottky barrier diode (SBD) is formed by the junction of a metal with a semiconductor and is very useful in various applications like rectifiers, filters, and integrated circuits.1−4 The SBD can include a native thin interface between the metal and the semiconductor that converts the MS structure into a metal–interlayer–semiconductor (MIS) structure.5−8 The performance, stability, and reliability of the SBD are significantly influenced by this interlayer, which is crucial for the regulation of electrical parameters.9−12 Lately, there has been an increasing interest in SBD that includes a carbon nanostructured layer inserted between a metal and semiconductor. Carbon-based nanostructures,13 in particular graphene quantum dots (GQDs),14 have had a profound impact on several fields including optoelectronic devices, biosensors, and energy conversion systems.15GQDs exhibit remarkable properties, including fluorescence properties, biocompatibility, versatility in synthesis from various organic precursors, low toxicity, abundant functional groups, tunable emission wavelength, photostability, effortless surface modification, and chemical inertness.16,17GQDs are functionalized in various ways to extend their application in various fields.18 The functionalization of GQDs is performed through doping with heteroatoms or by forming composites with inorganic materials or polymers and can alter their optical, chemical, and electronic properties.19−23 Nanocomposites of GQDs functionalized with polyethylenimine (PEI-GQDs) have been synthesized and used for optoelectronic applications9,24 as well as for cancer theragnostic and nucleic acid delivery.25−28 A review of the literature shows interesting and impressive results from Schottky diodes that exploit rare earth elements (REEs)-doped GQDs-based materials. REEs are an excellent choice for doping GQDs. They lead to the creation of hybrid materials that combine the favorable properties of GQDs and REEs to enhance their luminescence properties, applicability, and quantum yield, opening the door to a wide range of practical and technological applications.9 For instance, Orhan et al. fabricated a Gd-doped PEI-functionalized N-doped GQDs (Gd-dopedPEI-NGQDs) nanocomposite diode with enhanced electrical properties and high photoluminescence (PL) quantum yield (PLQY).9 Lanthanum (La)-based materials have been proven to be particularly promising. La is the first REE of the lanthanide series and is arranged in the [Xe]5d16s2 configuration with 57 electrons. The luminescence of lanthanides and their compounds has been used in many technological applications, such as color televisions, fluorescent lamps, energy-saving lamps, cameras, and telescope lenses.29,30 Lanthanide luminescent lifetimes are typically on the millisecond time scale, exceeding those observed for organic fluorophores, and may be useful for time-gated monitoring applications.17,31 Shao et al.32 showed that the performance of diamond SBD can be significantly improved by inserting an ultrathin lanthanum hexaboride (LaB6) layer followed by a rapid thermal annealing (RTA) treatment. Liu et al.33 investigated the electrical and hydrogen sensing properties of a Schottky diode based on a Pd/La-WO3/SiC structure, indicating that the presence of La significantly improves the Schottky diode hydrogen sensitivity.

Despite its technological potential, La(OH)3 doping of functionalized GQDs has still been poorly explored and requires further investigation.

In this study, La(OH)3NPs-dopedPEI-NGQDs nanocomposites were prepared by a green method in a single step from the reaction of La(NO3)3 with PEI-NGQDs in a water bath at 90 °C, and Fourier-transform infrared spectroscopy (FT-IR), ultraviolet–visible spectroscopy (UV–vis), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The electrical characteristics and the photodetection of the SBDs obtained by depositing the La(OH)3NPs-dopedPEI-NGQDs nanocomposite onto n-type Si are investigated as a function of the illumination intensity. It is shown that the La(OH)3NPs-dopedPEI-NGQDs/Si diodes achieve good rectification and responsivity and are promising for optoelectronic applications.

2. Materials and Methods

2.1. Materials

All chemicals were obtained from commercial sources and used without further purification. The citric acid (CA), La(NO3)3·6H2O, and polyethylenimine (PEI) (Mw: 1300, 50 wt % in H2O) were purchased from Sigma-Aldrich. The synthesized nanocomposite solutions were characterized by using complementary methods. Infrared absorption (IR) spectra were obtained from a PerkinElmer BX II FT (Fourier Transmission)-IR spectrometer on KBr discs. The UV–visible spectra were measured by using a PG Instruments T+80 UV–visible spectrometer. XPS studies of the PEI-NGQDs and La(OH)3NPs-dopedPEI-NGQDs nanocomposite materials were performed using the PHI ESCA system equipped with an Mg Kα photon source (hν = 1253.6 eV) and a hemispherical analyzer. Binding energy data were calibrated using the Ag 3d5/2 signal peak (368.3 eV) obtained by a small silver dot deposited onto each sample. CTEM analysis was carried out by an FEI Technai G2 Spirit BioTwin, 120 kV electron microscope. A specimen was prepared by sonification of the La(OH)3NPs-dopedPEI-NGQDs nanocomposite solution in DI water. A single droplet of the solution was dropped onto the carbon film-supported copper grid. The specimen was dried and analyzed. We finally examined the photodetection performance of the fabricated diode. A continuum white light source (NKT Photonics – SuperK COMPACT) was used to investigate the sensitivity (S), responsivity R, and specific detectivity (D*) of the photodetector under illumination in the range 22–110 mW cm–2. La(OH)3NPs-dopedPEI-NGQDs nanocomposites were morphologically characterized by high contrast transmission electron microscopy (CTEM) (FEI Technai G2 Spirit BioTwin) at an accelerating voltage of 120 kV.

2.2. Synthesis of PEI-functionalized N-doped GQDs(PEI-NGQDs) and La(OH)3NPs-dopedPEI-NGQDs nanocomposites

PEI-functionalized N-doped GQDs were successfully synthesized by a hydrothermal process, which is a green method.9,34,35 The CA (1.80 g, 8.57 mmol) and PEI (3.71 g, 2.85 mmol) were dissolved in 50 mL of deionized water in a Teflon container and placed in an autoclave. The autoclave was kept in an oven at 200 °C for 18 h. The suspension products in the autoclave cooled to room temperature were centrifuged at 12000 rpm for 10 min, and nanoparticles were collected. The collected PEI-NGQDs nanoparticles were washed twice with deionized water and once with ethanol. The obtained PEI-NGQDs were dried in a vacuum oven and stored in a desiccator. The synthesis of PEI-NGQDs is also detailed in our previous work.9,34,35

For the synthesis of La(OH)3 nanoparticles and their nanocomposites, a solution of PEI-NGQDs (1 g) in 100 mL water was added to a 250 mL round-bottom flask. Then, 20 mL of a 0.1 M La(NO3)3 solution was added into the PEI-NGQDs mixture. The mixture was heated at 90 °C for 2 h to complete the transformation process of La(NO3)3 into La(OH)3 nanoparticles from lanthanum(III)nitrate and PEI-NGQDs to form La(OH)3NPs-dopedPEI-NGQDs nanocomposites. La(OH)3NPs-dopedPEI-NGQDs nanocomposite solutions then were cooled to room temperature, the suspension products were centrifuged at 12000 rpm for 10 min, and the supernatant was collected. La(OH)3NPs-dopedPEI-NGQDs nanocomposites were washed once with deionized distilled water and ethanol. The nanocomposites were stored in a desiccator for later use. Interestingly, while La(0) nanometal particles (LaNPs) were expected to be formed in the reaction, La(NO3)3 was completely converted to La(OH)3 by the catalytic effect of PEI-NGQDs in an aqueous medium. PEI-NGQDs do not reduce La(III) to La(0) but convert it to La(OH)3 via PEI-ammonium hydroxides formed in an aqueous medium. PEI-NGQDs acted both as catalysts for the formation of La(OH)3 and as stabilizers for the nanoparticles formed. In a literature study, it was reported that La(OH)3 was synthesized from Li(NO3)3 in a single step in the presence of hexamethylenetetramine (HMTA) at 95 °C in an autoclave for 8 h.36 The synthesis of La(OH)3-dopedPEI-NGQDs nanocomposites is presented in Scheme 1.

Scheme 1. Synthesis of La(OH)3NPs-dopedPEI-NGQDs Nanocomposites.

Scheme 1

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2.3. Fabrication of the La(OH)3NPs-dopedPEI-NGQDs/n-Si Diode

For the fabrication of the diode, n-type Si (1–10 Ωcm, 350 μm thickness) wafers (100) were used. Following cleaning procedures, Au (99.999% pure) with 150 nm thickness was sputtered on the unpolished surface of the n-Si wafer to form an ohmic contact.34,35 A spin-coating technique was then used to deposit the resulting La(OH)3NPs-dopedPEI-NGQDs nanocomposite solution onto the polished surface of the wafer. The spinning speed was 3000 rpm, and spinning time was locked for 30 s. The resulting thin film thickness was 30 nm. Finally, circular Au contacts with a thickness of 150 nm were formed on La(OH)3NPs-dopedPEI-NGQDs – n type-Si by sputtering using a circular metal mask with a diameter of 0.5 mm. Figure 1 shows the La(OH)3NPs-dopedPEI-NGQDs–n-type Si diode structure. The fabrication of a PEI-NGQDs/n-Si control device proceeded similarly.

Figure 1.

Figure 1

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Schematic layout and energy band diagram of the La(OH)3NPs-dopedPEI-NGQDs/n-type Si heterojunction.

3. Results and Discussion

3.1. FT-IR, UV–vis, XPS, and TEM Characterizations of PEI-NGQDs and La(OH)3NPs-dopedPEI-NGQDs

FT-IR spectra of the La(OH)3NPs-dopedPEI-NGQDs nanocomposites exhibit characteristic changes in the functional group frequencies when compared with the spectrum of the starting materials PEI-NGQDs (Figure 2). The OH, COOH, NH2, NH, C–H, COO+C=N, C=C, C–N, and C–O vibration bands are observed at 3551–3498–3446, 3367, 3320–3200, 3125, 3054, 2967–2839, 1646, 1559, 1454, and 1373 cm–1 of starting materials PEI-NGQDs, respectively. In the La(OH)3NPs-dopedPEI-NGQDs nanocomposites, the OH, NH2, NH, C–H, COO+C=N, C=C, C–N, and C–O vibration bands are observed at 3575–3503–3428, 3353, 3308–3208, 3147, 3046, 2949–2839, 1641, 1551, 1447, and 1385 cm–1, respectively. The COO and C=N absorptions overlapped in the FT-IR spectra of all materials. Compared to the starting compound PEI-NGQDs, in metal nanocomposites, OH, COOH, NH2, and NH absorptions shifted to a higher frequency, while C–H, COO+C=N, and C–N absorptions shifted to a lower frequency. The C–O vibration was observed at a higher frequency in La(OH)3NPs-dopedPEI-NGQDs nanocomposites than in PEI-NGQDs. The C–O vibration was observed at a higher frequency in La(OH)3NPs-dopedPEI-NGQDs than in PEI-NGQDs. Here, both the La–OH bond and the C–O bond were vibrated together at a high frequency. Interestingly, the frequency of carboxyl + imine absorption bands observed at 1646 cm–1 in PEI-NGQDs shifted to the lower frequency of 1641 cm–1 in La(OH)3NPs-dopedPEI-NGQDs nanocomposites. More interestingly, new peaks at 1043 and 840 cm–1 were observed in La(OH)3NPs-dopedPEI-NGQDs nanocomposites, which were not observed in PEI-NGQDs. This new peak is evidence of the formation of the La–O bond.37−39 These data show that lanthanum(III)’ nitrate is converted to La(OH)3.

Figure 2.

Figure 2

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FT-IR spectra of PEI-NGQDs and La(OH)3NPs-dopedPEI-NGQDs nanocomposites.

The absorption spectra of suspensions of PEI-NGQDs and La(OH)3NPs-dopedPEI-NGQDs nanocomposites in water are shown in Figure 3. One band assigned to n-π* transitions of C=O and C=N is observed at 355 nm in the UV–vis spectrum of La(OH)3NPs dopedPEI-NGQDs nanocomposites and at 345 nm in the UV–vis spectrum of PEI-NGQDs nanocomposites. The absorption edge of pure La(OH)3 was observed at approximately 250 nm.40 After the reaction of La(NO3)3 with PEI-NGQDs, the absorption of PEI-NGQDs was observed to shift to blue (345 nm) in the nanocomposite, indicating the successful formation of La(OH)3NPs dopedPEI-NGQDs nanocomposites. Also in the nanocomposite, the shoulder at 335 nm was attributed to La(OH)3 nanoparticles. The band gap values of pure La(OH)3,40PEI-NGQDs, and La(OH)3NPs-dopedPEI-NGQDs nanocomposites are 5.20, 3.10, and 2.90 eV, respectively. The band gap was found to be smaller in the La(OH)3NPs-dopedPEI-NGQDs nanocomposites, indicating that the nanocomposite is a better conductor.

Figure 3.

Figure 3

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UV–vis spectra of PEI-NGQDs (red curve) and La(OH)3NPs-dopedPEI-NGQDs (light blue curve) nanocomposites.

XPS was also used to characterize the synthesized nanomaterials. Figure 4 shows the survey spectra of La(OH)3NPs-dopedPEI-NGQDs (black curve) and PEI-NGQDs (red curve). No contaminant species are detectable within the sensitivity of the technique.

Figure 4.

Figure 4

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XPS survey spectra of La(OH)3NPs-dopedPEI-NGQDs (blue curve) and PEI-NGQDs (red curve). The inset shows the magnification of the La 3d region.

From the La 3d, C 1s, O 1s, and N 1s peaks, it is possible to obtain the exact composition of the nanocomposite by calculating the atomic concentration of the individual species using respective sensitivity factors. For the La(OH)3NPs-doped PEI-NGQDs sample, the atomic percentage concentrations of 68%, 24%, 7%, and 1% were obtained for C, O, N, and La, respectively. The inset of Figure 4 displays a magnification of the La 3d region confirming the formation of La(OH)3.41

In conclusion, from FTIR, UV–vis, and XPS data, it can be said that La is La3+ in the La(OH)3NPs-dopedPEI-NGQDs nanocomposite.

TEM image of La(OH)3NPs doped PEI-NGQDs nanocomposites is shown in Figure 5. TEM analysis shows that La(OH)3NPs in the nanocomposite have a spherical structure, with an average size of 6–20 nm. PEI-NGQDs appear as distorted spherical elongated shapes with smaller sizes among La(OH)3NPs. It can also be said that the particles form random and partial aggregates.

Figure 5.

Figure 5

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TEM image of the La(OH)3NPs-dopedPEI-NGQDs nanocomposite.

3.2. Electrical Characterization

Current–voltage (I–V) measurements in the dark and under light were performed to determine the electrical properties of the La(OH)3NPs-dopedPEI-NGQDs/n-Si Schottky diode. Thermionic emission (TE) theory2,42−44 and Cheung’s method45 were used to estimate the parameters of the diode. The relationship between the TE current and the applied voltage is expressed as follows:

3.2. 1

where I0, n, V, k, q, IRs, and T are the reverse saturation current at zero bias, ideal factor, applied bias voltage, Boltzmann constant, electronic charge, voltage drop across series resistance (Rs), and temperature in Kelvin, respectively. The reverse saturation current at zero bias, I0, can be obtained from the lnI intercept of the straight-line fitting the linear part of the ln I–V curve at low forward bias, where the effect of the series resistance Rs is negligible. Then, I0 can be used to extract the Schottky barrier height φB from the equation:

3.2. 2

where A* is the Richardson constant, that is 112 A cm–2 K–2 for n-type Si at room temperature, and A is the area of the heterojunction.

Similarly, the ideality factor n can be obtained from the slope of the semilog I–V curve at low forward bias as

3.2. 3

The Cheung method offers an alternative to find n, Rs, and φB using the higher forward bias region of the I–V characteristic, where the effect of Rs is not negligible and ln I–V shows a downward curvature.

The Cheung method utilizes the following two current- and voltage-dependent functions:

3.2. 4
3.2. 5

The calculations of the Cheung functions, henceforth referred to as Cheung-1 (dV/d lnI) and Cheung-2 (H(I)), are described in detail elsewhere.46−48

Figure 6a–d reports the plots of ln I vs V, rectification ratio RR vs light intensity, ln Iforward vs ln Vforward, and φB vs n for the La(OH)3NPs-dopedPEI-NGQDs/n-Si diode in the dark and under different illumination intensities (P) from 22 to 110 mW cm–2. Figure 6a shows rectifying ln I–V characteristics with the current increasing for an increasing illumination intensity. Remarkably, the current reaches a good saturation at reverse biases both in the dark and under illumination. The inset of Figure 6a shows the linear region used to extract the diode parameters using the TE method. The increased current under illumination is due to electron–hole pair photogeneration. Under reverse bias, a photocurrent growing with the enhanced illumination intensity is observed because the electron–hole pairs, photogenerated in the extended depletion region of the La(OH)3NPs-dopedPEI-NGQDs/n-Si heterojunction, are efficiently separated by the strong electric field (internal + external electric fields). Conversely, in forward bias, the weakened electric field (internal – external electric fields) and the reduced depletion region result in a current under illumination that is indistinguishable from the dark current. Hence, the rectification ratio, RR(V) = I(V)/I(−V), decreases with increasing illumination, as shown in Figure 6b.

Figure 6.

Figure 6

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(a) ln I vs V, (b) RR vs light intensity, (c) ln Iforward vs ln Vforward, and (d) ϕB vs n from TE theory for the La(OH)3NPs-dopedPEI-NGQDs nanocomposite diode under different illumination intensities.

Figure 6c shows the ln Iforward vs ln Vforward plot under different illumination intensities. This plot gives essential clues about the dominant current conduction mechanisms in the forward bias. The linear parts of the double-logarithmic current–voltage curve correspond to power law relationships (Iforward ∝ Vmforward), with the exponent ″m″ indicating different conduction mechanisms. At high biases, for V > 0.5 V, the value of m approaches 2, indicating a space-charge-limited current trend.49

The basic parameters of the La(OH)3NPs-dopedPEI-NGQDs/n-Si diode obtained from TE theory and Cheung-1 or Cheung-2 functions are shown in Table 1 as a function of the illumination intensity. Notably, Table 1 shows that the ideality factor n decreases while the Schottky barrier ϕB increases with the growing light intensity. Figure 6d shows that there is an anticorrelation between n and φB. This behavior can be attributed to spatial inhomogeneities of the Schottky barrier, which may result from lattice defects and/or surface impurities.50 At low illumination, electrons cross the barrier mainly at the minima of the barrier height, thus resulting in lower average φB and higher n.51−53

Table 1. Basic Diode Parameters of the La(OH)3NPs-dopedPEI-NGQDs Nanocomposite Diode.

TE
 Cheung-1 (dV/dlnI)
 Cheung-2 (H(I))
P(mW.cm–2) Rs(kΩ) RR I0(nA) n φB(eV) n Rs1( kΩ) φB(eV) Rs2(kΩ)
0 4.8 2760 34.6 2.80 0.737 3.31 6.9 0.698 4.3
22 4.8 1435 32.5 2.76 0.738 3.46 6.9 0.691 4.1
44 4.7 746 30.5 2.70 0.740 3.62 6.9 0.686 4.0
66 4.8 311 20.1 2.52 0.751 3.57 6.9 0.688 4.0
88 4.8 219 17.9 2.49 0.754 3.48 6.9 0.691 4.0
110 4.8 152 15.2 2.43 0.758 3.46 6.9 0.692 4.0
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The n and ϕB values obtained from the TE method are 2.80 and 0.737 eV in the dark. Cheung-1 and Cheung-2 plots of the La(OH)3NPs-dopedPEI-NGQDs nanocomposite diode are shown in Figures 7a,b. The series resistances Rs of the structure are obtained from the slopes of these plots, and in the dark, results are 2.1 kΩ (Cheung-1) and 1.7 kΩ (Cheung-2), respectively. Using eq 5, the ideality factor n = 3.31 is extracted from the intercept point of the y-axis of the plot. Substituting this n value into eq 5, and ϕB = 0.698 eV is obtained. These values are consistent with TE ones.

Figure 7.

Figure 7

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(a) Cheung-1 and (b) Cheung-2 plots of the La(OH)3NPs-dopedPEI-NGQDs nanocomposite diode at different intensities of illuminations.

Berktaş et al. investigated the I–V34 and C/(G/ω)-V35 properties of the PEI-NGQDs nanocomposite-based diode across a frequency range of 1 kHz to 2 MHz and voltage range of −3 to +7 V. Orhan et al. examined the electrical properties and photoluminescence quantum yield (PLQY) of PEI-NGQDs doped with the rare earth element Gd.9 The PLQYs of GdNPs PEI-NGQDs (35.96%) were compared with those of the Gd-free PEI-NGQDs sample (7.60%), revealing a 470% increase in quantum efficiency for the Gd-doped sample. Additionally, it has been noted that the Gd-free PEI-NGQDs sample exhibits a good rectification ratio (RR: 2.8 × 104, ± 5 V), whereas, after Gd doping, the diode demonstrates ohmic behavior (RR: 14, ± 5 V). Furthermore, the Gd-doped PEI-NGQDs structure displayed no negative capacitance (NC) behavior across any frequency range,34 while NC behavior at low frequencies was observed in the Gd-free PEI-NGQDs sample.35

A comparison of diode parameters for PEI-NGQDs,34GdNPs-dopedPEI-NGQDs,9 and La(OH)3NPs-dopedPEI-NGQDs-based diodes in dark conditions at 300 K is presented in Table 2. Berktaş et al.34 reported a rectification ratio (RR) of 2.8 × 104 for the undoped PEI-NGQDs diode at ±5 V. The findings indicate that the RR of the undoped PEI-NGQDs diode is 10-fold higher than that of the La(OH)3NPs-dopedPEI-NGQDs diode (RR = 2.8 × 103 at ±2 V). Berktaş et al.34 observed an ohmic response with a low RR (14 at ±5 V) for a Gd-doped PEI-NGQDs diode. Comparing the undoped structure with the lanthanum doped diode, it was observed that the lanthanum doped sample had a lower n value approaching the ideal diode value and no significant change (2%) in barrier height for both structures.

Table 2. Comparison of Diode Parameters of PEI-NGQDs, GdNPs-PEI-NGQDs, and La(OH)3NPs-PEI-NGQDs-Based Diodes.

  diodes
  PEI-NGQDs(34)
 GdNPsPEI-NGQDs(9)
 La(OH)3NPs-PEI-NGQDs in the present study
methods n ϕB(eV) RR Rs(kΩ) n ϕB(eV) RR Rs(kΩ) n ϕB(eV) RR Rs(kΩ)
TE 3.71 0.76 2.8 × 104   4.9 0.61 14 3.85 2.8 0.74 2.8 × 103 4.8
Cheung-1 (dV/dln I) 9.2 - - 143 11.6 - - 2.63 3.31 - - 6.9
Cheung-2 (H(I)) - 0.66 - 110   0.58   2.35 - 0.70 - 4.3
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Surface states (Nss) and their distribution play an important role in the current transport mechanism. These states, which can have different origins and can depend on the illumination intensity, are studied using the method presented by Card and Rhoderick54 and obtained from the relation

3.2. 6

where q is the charge of an electron, εs and εi are the dielectric permittivity of the semiconductor and interlayer, respectively, WD is the depletion layer width, n(V)is the voltage-dependent ideality factor, and δ is the interfacial layer thickness. The energy levels of the surface states (Ess) are calculated relative to the edge of the conduction band (Ec) for n-type Si and are given by

3.2. 7

where φe is the effective barrier height. This is provided by

3.2. 8

Figure 8 shows the plots of Nss as a function of Ec – Ess obtained from the above equations. The surface states exhibit a decrease that is nearly exponential as the energy difference Ec – Ess increases. However, with the growing illumination, the concentrations begin to decrease. The illumination that generates electron–hole pairs at the interface also leads to a decrease in the surface states, consistently with the observed decreasing ideality factor.55−57

Figure 8.

Figure 8

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Nss vs Ec–Ess plots of the La(OH)3NPs NPs-dopedPEI-NGQDs nanocomposite diode under different illumination intensities.

3.3. Photodetector’s Figures of Merit

First, the transient photocurrent (Itransient) characteristics were studied as a function of the increasing light intensity with 30 s long light pulses at zero bias (Figure 9a). The Itransient increases rapidly with each time the surface is illuminated. As the illumination intensity increases, Itransient shows stronger time dependence that can be attributed to the distribution of Nss and their impact on photogeneration.58−61

Figure 9.

Figure 9

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(a) Itransient vs t, (b) sensitivity vs V, (c) responsivity vs V, and (d) detectivity vs V for the La(OH)3NPs-dopedPEI-NGQDs nanocomposite diode under different illumination intensities.

The ratio of photocurrent to dark current, S = (Ilight– Idark)/Idark, is defined as the sensitivity (S) of an optoelectronic device. The S curves of the diode are shown in Figure 9b from −2 to 0 V bias under different illumination intensities (P). S increases as the illumination intensity increases at a given reverse bias. The La(OH)3NPs-dopedPEI-NGQDs nanocomposite diode shows an increasing sensitivity from 0.94 to 17.37 under 22 and 110 mW cm–2 at −2 V, respectively. The S versus P plots are given for certain reverse voltages in Figure 10a. As can be seen in Figure 10a, the fabricated La(OH)3NPs-dopedPEI-NGQDs nanocomposite diode exhibits photosensitivity, and the value of photosensitivity increases with illumination intensity.

Figure 10.

Figure 10

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(a) R vs P plots, (b) S vs P plots, and (c) D vs P plots at certain reverse voltages for La(OH)3NPs-dopedPEI-NGQDs nanocomposite diode under different illumination intensities.

The responsivity is defined as the ratio of the photocurrent density (Jph =Jlight– Jdark) to the incident light intensity and is a measure of the ability of a photodiode to convert incident light into an electrical current:

3.3. 9

The variation of R as a function of P in reverse bias is shown in Figure 9c. Like S, R increases for the growing illumination intensity and the reverse bias. The La(OH)3NPs-dopedPEI-NGQDs nanocomposite diode shows an increasing responsivity from 0.8 to 7.7 mAW–1 under 22 and 88 mW cm–2 illuminations at −0.3 V, respectively. The R versus P plots are given for certain reverse voltages in Figure 10b. R increases with the illumination intensity. The variation of R with the growing reverse bias is due to the enhancing separation of photogenerated electron–hole pairs in the widening depletion layer of the nanocomposite heterojunction.55−57

Specific detectivity (D*) is an essential parameter for optoelectronic devices, describing the smallest detectable signal, and can be estimated byeq 1059−63

3.3. 10

where Jdark is the current density measured in the dark and D* is in Jones units (Jones = cmHz0.5W1–). D* is shown in Figure 9d from −2 to 0 V bias at different illuminations. D* increases from 1.8 × 108 to 9.8 × 108 Jones under 22 and 88 mW cm–2 illuminations at −0.3 V, respectively. The D versus P plots are given for certain reverse voltages in Figure 10c. As can be seen in Figure 10c, the diode shows good detectivity, and D increases with illumination intensity.

Conclusions

In this work, we have successfully synthesized La(OH)3NPs-dopedPEI-NGQDs nanocomposites were prepared by a green method in a single step from the reaction of La(NO3)3 with PEI-NGQDs in a water bath at 90 °C. The nanocomposites are characterized by FT-IR, UV–vis, XPS, and TEM analyses. We have fabricated a heterojunction by depositing the La(OH)3NPs-dopedPEI-NGQDs nanocomposite over n-type Si. We have investigated the electrical behavior and the photoresponse of the heterojunction as a function of the illumination intensity, demonstrating that the device is rectifying and photosensitive. Using the TE and Cheung function method, we have estimated the diode parameters and shown that the Schottky barrier φB increases while the ideality factor n decreases for increasing illumination intensity. As a photodetector, the La(OH)3NPs-dopedPEI-NGQDs nanocomposite diode shows an appreciable responsivity of 3.9 × 10–3 AW–1 under 22 mW cm–2 at −0.3 V and maximum detectivity of 8.7 × 108 Jones under 22 mW cm–2 at −0.5 V. This study provides insights into the fabrication and the electrical and photodetection properties of La(OH)3NPs-dopedPEI-NGQDs/n-Si heterojunction nanocomposite devices.

Acknowledgments

E.O. is grateful to the Gazi University Scientific Research Council (BAP) for the financial support of this research under project FGA-2022-8252. A.D.B. and A.K. acknowledge the financial support of the European Union’s REACT-EU PON Research and Innovation 2014-2020 project, Italian Ministerial Decree 1062/2021, and the University of Salerno, grant number ORSA235199.

Data Availability Statement

All data included in this study are available upon request by contact with the corresponding author.

This study was funded by the Scientific Research Council of Gazi University (BAP) grant number FGA-2022–8252, the REACT-EU PON Research and Innovation 2014–2020 project of the European Union, the Italian Ministerial Decree 1062/2021 and the University of Salerno, grant number ORSA235199. Open access is funded by The Anatolian University Libraries Consortium (ANKOS) in Türkiye.

The authors declare no competing financial interest.

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Data Availability Statement

All data included in this study are available upon request by contact with the corresponding author.

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