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. 2026 Sep 21;8(21):6427–6437. doi: 10.1039/d6na00631k

Hydrothermally synthesised Gd2Te4O11 nanorods for broadband visible light photodetection and NLO applications

Manoranjan Sahoo a, Prabhukrupa Chinmay Kumar a, Ramakanta Naik a,✉
PMCID: PMC13636259  PMID: 42835976

Abstract

Rare earth tellurium oxides belong to a wide bandgap group, which makes them prominent candidates for optoelectronic applications. Gadolinium tellurate (IV) (GTO) samples with varying concentration ratios of Gd and Te were synthesised through a hydrothermal method. All samples confirmed the Gd2Te4O11 structure with a monoclinic crystal system. Nanorods are observed in most samples, and EDX confirmed the presence of the constituent elements. The XPS study was carried out to obtain the chemical composition and oxidation states. Optical studies of materials showed band gap values lying between 4.1 eV and 4.3 eV. From the photoresponse study, it is clear that the performance of all GTO photodetector devices (PDs) is good. The nonlinear optical (NLO) study confirmed that the nonlinear refractive index dominated over two-photon absorption for all samples, with a maximum nonlinear refractive index of 3.305 × 10−5 cm2 W−1 and a nonlinear optical susceptibility of 38.811 × 10−4 (esu). All the samples showed a high figure of merit (FOM), with a maximum value of 195.789. This makes GTO samples suitable for all nonlinear optical devices and optical switching applications.

Hydrothermal synthesis of GTO nanorods, followed by structural characterization and evaluation of their nonlinear optical and broadband photodetection properties.graphic file with name d6na00631k-ga.webp

1. Introduction

Lanthanide-containing materials have emerged as a new variety of materials due to their applications in many areas. These areas mainly include electrical, optical, and chemical properties. Lanthanide-containing materials are successfully used in lasers, wide-gap electroluminescent devices, LEDs, and scintillators.1–5 Comparatively less attention has been given to rare-earth tellurates. These materials belong to the wide-bandgap semiconducting group. A wide bandgap potentially makes them useful in a broad range of optical and electrical applications. Tellurium is an energy-critical element. Among chalcogens, the structure, properties, and reactivity make tellurium unique.6,7 A systematic study on the spectroscopic properties of trivalent lanthanide ions has been performed for rare-earth tellurium oxides.8–13 Rare-earth tellurium oxides are a unique class of materials due to their applications in diverse areas. The general formula RE2Te4O11 belongs to a large family of rare-earth tellurium oxides, simply referred to as rare-earth tellurates (iv). The structure of digadolinium tellurate (iv) belongs to the RE2Te4O11 group. The three-dimensional structure of Gd2Te4O11 is formed from an interconnected network between ∞2[Gd2O1014−] and ∞2 [Te4O116−].14

Most of the past studies were on the synthesis and structure, with a few investigating the optical, electronic, photoluminescent, thermal, and magnetic properties of oxytellurides and oxotellurates of the rare-earth group.15–24 Recently reported studies of our group were on photodetection and NLO applications of ternary oxytellurides and oxotellurates.25,26 However, still not so much attention is given to the RE2Te4O11 group. Only a few studies of this group have been done. This might be due to difficulty in the synthesis of the material. In ternary tellurates (iv), the oxidation state of tellurium is 4. The family of this group provides a rare 2D honeycomb lattice magnet for studying exotic magnetic states, and the inverse magnetocaloric effect at low temperature was reported in 2025.27 The near IR luminescence spectrum of Er2Te4O11 showed that the luminescence lifetime for the emission band at 1538 nm is 0.275 ms.28 Er2Te4O11 dispersed tellurite glass showed high luminescence upconversion efficiency, useful for solar IR concentrators and NIR sensors.29 The data fit showed a Curie–Weiss behavior, and the values of Weiss constants were negative for Ln2Te4O11 (Ln = Gd, Nd, Ho) in the temperature range from 50 to 300 k. Quasi-Curie-like behavior was observed for Gd2Te4O11.9 In addition to a wide band gap, Pr2Te4O11 showed Curie–Weiss paramagnetic behavior.14 The performance of Yb3+/Er3+ co-doped Gd2Te4O11 has good reliability in optical temperature measurements.30

In most of the reported studies, the materials were synthesised by a solid–state reaction mechanism. As these materials are new and possess a wide range of band gaps, the compounds of this group are encouraged to be further investigated for their optoelectronic and nonlinear optical properties. We tried the hydrothermal method for the synthesis process, as this approach is cost-effective and eco-friendly. Also, highly phase-pure crystalline materials are produced in this process.

Rare earth tellurium oxide (iv) Gd2Te4O11 nanoparticles were synthesised via a hydrothermal route by varying the tellurium concentration with respect to Gd in this work. This work aims to investigate how the incorporation of Te affects the structure, morphology, band gap, and, most importantly, the interaction of light with matter. The first investigation of tunable broadband photodetection and nonlinear optical behavior of the GTO samples makes this work novel. The third-order nonlinear susceptibility of the GTO samples is supported by both experimental and theoretical insights. This research work focuses on a comprehensive understanding of Te-induced band modulation and its effect on the photoresponse behavior and nonlinear optical properties of ternary rare-earth oxotellurates (iv). The performance of the GTO nanomaterial demonstrates its potential for future generation optoelectronic and photonic device applications.

2. Experimental

2.1. Materials

For the synthesis, gadolinium(iii) nitrate hexahydrate (Gd (NO3)3·6H2O 99.99%; SRL Chemicals) and sodium tellurite (Na2TeO3, 98%; Oxford Lab Fine Chem) were used as source materials of gadolinium and tellurium, respectively. Hydrazine hydrate (N2H6O, 80%; Sigma Aldrich) was used as a reducing agent, and sodium hydroxide (NaOH, 85%; SD Fine Chemicals) was used as a mineralizer. Deionised (DI) water was used as a common solvent in the synthesis process.

2.2. Synthesis process

All samples have been prepared by varying gadolinium and tellurium concentrations using a hydrothermal method. The source materials of Gd and Te were taken in molecular ratios of 1 : 1, 1 : 2, and 1 : 3. We fixed the molar concentration of Gd at 4 mmol, with a mass of 144.435 mg, while varying the molar concentration of Te at 4 mmol (70.90 mg), 8 mmol (141.80 mg), and 12 mmol (212.70 mg). After the stoichiometric calculation, all the reactants were taken and added to 75 ml of DI water placed in a beaker. Then, the aqueous solution was stirred at 700 rpm for one hour at room temperature (∼35 °C). The stirred homogeneous solution was placed in a 100 ml Teflon-lined autoclave and heated in an oven at 200 °C for 24 hours. After 24 hours, the autoclave was allowed to reach ambient temperature. The reaction mixture was then centrifuged several times to collect the residue. This process was followed by multiple washes with DI water and ethanol to avoid the presence of impurities and unreacted substances. Finally, the powder sample was collected after drying in a vacuum oven at 60 °C for 24 hours. The Gd : Te precursor molar ratios of 1 : 1, 1 : 2, and 1 : 3 correspond, respectively, to samples GTO-1, GTO-2, and GTO-3 (Scheme 1).

Scheme 1. Schematic of the hydrothermal synthesis process of the Gd2Te4O11 material.

Scheme 1

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2.3. Characterization

Several characterization studies of the prepared samples were performed to gather more information, such as structure, morphology, optical behavior, and photoresponse study. Structural information of the samples was collected using an X-ray Diffractometer Bruker D8 Advance instrument with Cu Kα radiation of 1.54 Å wavelength. The operation was carried out at 1200 watts (current at 30 mA and voltage at 40 kV) with a step size of 0.020 s−1 and 2θ varying from 10° to 70°. The detailed surface structure and chemical composition of the samples were determined using a JEOL-JSM-7610F plus FESEM unit combined with energy-dispersive X-ray (EDX) spectroscopy. Raman spectra were obtained using a RENISHAW inVia (Model RE 04) confocal microscope enclosure system fitted with an argon laser with an excitation wavelength of 532 nm. X-ray photoelectron spectroscopy (XPS) was performed using an Axis Ultra instrument, Kratos Analytical, UK. A JASCO V-770 UV-Vis-NIR spectrometer was used to obtain the diffuse reflectance spectra (DRS). Photoresponse studies of the GTO samples were carried out using a Keithley 2450 Source Meter Unit (SMU), as detailed in the SI. The Z-scan study conducted for the nonlinear optical properties is also described in the SI file.

3. Results and discussion

3.1. XRD and Raman study

The XRD pattern of each synthesised sample is displayed in Fig. 1(a). After matching with the peaks of the Inorganic Crystal Structure Database (ICSD), most of the observed peaks matched with the ICSD card (413657). This belongs to Gd2Te4O11 with the monoclinic crystal system and C2/c (15) space group. The lattice parameters of this phase are a = 12.4918 A0, b = 5.1507 A0, and c = 16.1326 A0. We calculated the crystallite size by using the Scherrer equation. The crystallite size was estimated from the measured FWHM of the diffraction peaks using the Scherrer equation. No separate instrumental-broadening correction was applied; therefore, the calculated values represent Scherrer-estimated crystallite sizes based on the observed peak broadening. The formulas for crystallite size, dislocation density, and lattice strain are given by

3.1. 1
3.1. 2
3.1. 3

Fig. 1. (a) XRD patterns of the GTO samples matched with the Gd2Te4O11 ICSD (413657) and (b) Raman spectra of individual samples.

Fig. 1

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The calculated values of average crystallite size, dislocation density, and lattice strain are displayed in Table S1 for all samples. The average crystallite size of the GTO samples increases while the dislocation density and lattice strain decrease with an increase in Te concentration.

The Raman spectra of the GTO samples are shown in Fig. 1(b). Raman spectroscopy is highly sensitive to changes in the tellurium–oxygen environment, as the Te–O bond is highly polarisable. Distinct Raman peaks at 92.36 cm−1, 120.67 cm−1, and 140.81 cm−1 correspond to the first order Raman modes E(1), A1, and E(2) of trigonal Te in the low-frequency region.31 Another peak below 300 cm−1 at 267.19 cm−1 may be assigned to both TeO3 tp and Gd–O bonds.32

3.2. XPS analysis

XPS measurement of one of the GTO samples is carried out to determine the chemical composition and bonding states. The complete XPS survey spectrum of the GTO-3 sample in the energy range 0–1300 eV is shown in Fig. 2(a). The intense peaks of Gd 4d3/2 and 4d5/2 are seen at 147.2 eV and 141.6 eV, respectively, in Fig. 2(b). These peaks represent the trivalent state of Gd. The peaks of Gd in the 4d region may be due to the 4d – 4f exchange interaction. A single peak of O 1s is observed at 530.5 eV, as shown in Fig. 2(c). This may be due to oxygen atoms bonded with Gd. The peaks of Te 3d3/2 and 3d5/2 are seen at 586.2 eV and 575.8 eV, respectively, as displayed in Fig. 2(d). These peaks represent the Te4+ (tellurite) valence state.

Fig. 2. XPS spectra of the GTO-3 sample: (a) entire spectra survey and (b–d) high-resolution XPS spectra of individual Gd, O, and Te elements.

Fig. 2

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3.3. UV spectroscopy

DRS is used to determine the band gap of a material, and this provides information regarding the optical properties of the material. Diffuse reflectance spectra of all the samples are displayed in Fig. S2(a) (SI file).

In terms of reflectance, the Kubelka–Munk function F(R∞) is expressed as33

3.3. 4

Here, R∞ represents the relative reflectance of the material. K and S are the absorption coefficient and scattering coefficient, respectively.

The band gap of the prepared GTO samples is obtained from the Tauc plot. The relationship between F(R∞) and the band gap (Eg) is

F(R∞)hν = B(hν − Eg)n 5

B is the Tauc parameter or Tauc slope that determines the transition probability and material disorder. Eg is the optical band gap, and n is the transition exponent that shows the nature of the electronic transition. For a direct allowed transition, n is 1/2, and for an indirect allowed transition, n is 2. For a direct forbidden transition and an indirect forbidden transition, the value of n is 3/2 and 3, respectively. Since the GTO material is a wide bandgap semiconductor, we may substitute 1/2 or 2 for n for the allowed transition. Here, we took n = 2 as GTO is considered to be an indirect bandgap semiconductor.30 The bandgap of each GTO sample is obtained by extrapolating the linear part of the curve, as shown in Fig. S2(b–d). The bandgap of GTO-1, GTO-2, and GTO-3 is 4.3, 4.1, and 4.2 eV, respectively. The decrease in the bandgap of GTO-2 and GTO-3 as compared to that of GTO-1 might be due to an increase in tellurium concentration.

The band gap does not vary monotonically with precursor Te content, but instead tracks the crystallite size and defect density obtained from XRD (Table S1). GTO-1, with the smallest crystallite size (39.6 nm) and the highest dislocation density/lattice strain among the three samples, shows the widest band gap (4.3 eV). GTO-2 has an intermediate crystallite size (48.6 nm) but exhibits the narrowest band gap (4.1 eV), which we attribute to Te-related defect/sub-band-gap states introduced by the higher relative Te content. GTO-3, despite having the largest crystallite size (64.8 nm) and the lowest dislocation density/lattice strain (i.e., the best crystallinity of the three), shows a slightly higher band gap (4.2 eV) than GTO-2, consistent with the lower defect-state density partially restoring the optical gap. This confirms that the net band gap of the GTO samples results from a combination of Te-content-related defect states and structural/strain effects rather than from the precursor stoichiometry alone.

3.4. FESEM and EDAX

The micrographs of all the synthesised GTO samples at different magnifications are shown in Fig. 3. The morphology of the prepared samples from the micrograph is mostly a nanorod (NR) like structure.

Fig. 3. Micrographs of GTO-1(a and d), GTO-2(b and e), and GTO-3(c and f) at different magnifications.

Fig. 3

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Fig. 4 shows EDX elemental mapping of the GTO samples. The combined mapping shows the presence of all the elements from Fig. 4(a). However, Fig. 4(b–d) confirm the presence of individual elements in the sample. The EDX spectrum of GTO-3 is shown in Fig. 4(e). This shows the presence of all the elements. EDX spectra confirming the presence of constituent elements in GTO-1 and GTO-2 are shown in Fig. S1. The elemental percentages of the GTO samples are presented in Table S2. However, it shows some deviation from the ideal Gd : Te : O stoichiometry across GTO-1, GTO-2, and GTO-3. This is attributable to (i) the semi-quantitative and surface-sensitive nature of EDX, whose signal is influenced by the nanorod morphology/orientation and by any surface-adsorbed residual precursor species (Na+, NO3−, and OH−) that are not fully removed during washing, and (ii) the different nucleation/growth kinetics of the Gd3+ and TeO32− species under hydrothermal conditions, which can lead to a local Gd : Te ratio in the crystallites that does not scale linearly with the precursor feed ratio.

Fig. 4. EDX elemental mapping of the GTO sample: (a) combined mapping and (b–d) individual elements Gd, O, and Te, and (e) EDX spectrum of GTO-3 showing the presence of the respective elements.

Fig. 4

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3.5. Photoresponse study

The behaviour of the GTO composite was studied using the PDs for broadband photodetection applications. The details of the photoresponse measurement were already mentioned in the Experimental section. The experiment for each was conducted several times to confirm the accuracy of the findings. The current–voltage characteristics of the GTO PDs are shown in Fig. 5 with variation of sweep voltage from −10 V to 10 V for both dark and light conditions. The value of current is taken on a logarithmic scale. Individual I–V plots of the PDs are taken on a linear scale, as displayed in Fig. S3. All the plots clearly indicate that the value of current is higher under illumination rather than under dark conditions. The difference in the value of current is due to the interaction of photons with the charge carriers of the material. The values of light and dark currents obtained from Fig. S3 are mentioned in Table 1. The performance of the PDs was obtained using key parameters such as photosensitivity, responsivity, detectivity, and noise equivalent power (NEP) in Table 1.34

Fig. 5. Logarithmic plot of I–V characteristics of PDs under both dark and light conditions of all samples: (a) GTO-1, (b) GTO-2, and (c) GTO-3, and combined I–V plots of all samples under (d) dark conditions and (e) light conditions.

Fig. 5

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Table 1. The photo-response performance indices of all GTO PDs.

Sample I L (µA) I D (µA) Photosensitivity (%) Responsivity (R) AW−1 Detectivity (D*) ×1010 Jones NEP (×10−11 WHz−1/2)
GTO-1 355.555 233.209 52.461 0.244 1.416 3.530
GTO-2 955.631 757.999 26.072 0.395 1.269 3.940
GTO-3 680.729 532.973 27.722 0.295 1.131 4.419
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Photosensitivity determines the change in current of the device under light conditions compared with dark conditions. Responsivity (R) measures the PD's efficiency to convert incident light into an electrical signal and is expressed in AW−1 units. Detectivity is the ability to detect a weak signal against its internal noise. The power of incident light that generates a signal equal to the noise of the detector is the NEP.33–35

3.5. 6
3.5. 7
3.5. 8
3.5. 9

In the above equations, IL is the net current under illumination, and ID is the total current in dark conditions. The difference between IL and ID (IL − ID) is the net photocurrent (Iph). ‘A’ is the active surface area of the PD. Pin is the incident power density of the source light. Power density is the power of incident light per unit area (A). Here, the active surface area (A) = 0.25 cm2 and Pin = 2 mW cm−2. ‘e’ is the electronic charge.

The values of the above parameters are presented in Table 1. The photosensitivity of GTO-1 PD is 52.461, which is higher in comparison to the other two PDs. This means that it is more efficient at converting incident photons into electron–hole pairs. As the value of dark current is minimum in GTO-1 PD among all the PDs, the sensitivity of GTO-1 is better. The minimum value of dark current may be due to the highest bandgap of GTO-1. Lowering the value of dark current produces a better signal-to-noise ratio by reducing the background noise. The corresponding values of responsivity, detectivity, and noise equivalent power of GTO-1 are 0.244 AW−1, 1.416 × 1010 Jones, 3.530 × 10−11 WHz−1/2, respectively. As the value of photocurrent of GTO-2 is better in comparison to the other samples, the responsivity of GTO-2 is higher, i.e., 0.395 AW−1. The lowest bandgap value of GTO-2 might be the reason for the increase in the value of both dark and light currents. The photodetection parameters of different materials are mentioned in Table S3.

The current vs. time (I ∼ T) and the response time characteristics of all PDs during rise and fall are shown in Fig. 6. Different parameters are presented in Table S4 under both rise and decay conditions.36 From Fig. 6(d–f), it is clear that the response is asymmetric. The photo-switching ratio of GTO-3 PD is better than that of all other PDs. The on-to-off ratio during rise is more than that during decay, which increases the performance of GTO-3. This will be useful in basic light-sensing applications. Response time plays a crucial role in high-speed device applications. From Fig. 6 and Table S4, it is observed that the rise time is shorter than the fall time for all PDs. GTO-1 PD shows a smaller rise time and larger decay time as compared to the other two. This suggests the usefulness of all PDs for high-speed device applications.

Fig. 6. Multicycle (a–c) and single-cycle (d–f) I–T curves, and the response time of each PD.

Fig. 6

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3.6. Z-scan study

The Z-scan was performed to determine the absorption coefficient (β), nonlinear refractive index (n2), and third-order nonlinear optical susceptibility (χ(3)) of the GTO-1, GTO-2, and GTO-3 samples. A 100 mW DPSS continuous-wave laser at 532 nm was used in a Z-scan setup to observe the NLO behavior of the samples. Open aperture (OA) and closed aperture (CA) scans were used simultaneously. The transmitted power of OA measurement was measured without the aperture at PD1, while CA measurements used an aperture before PD2.

3.6.1. Open aperture nonlinear absorption

OA Z-scan data are used to analyse the nonlinear absorption behavior of each GTO sample. Saturable absorption (SA) takes place when the transparency becomes maximum (peak) at Z = 0, and reverse saturable absorption (RSA) occurs if the transparency is minimum (valley) at Z = 0. The OA Z-scan data, along with the theoretical fit of each sample, are plotted in Fig. 7(a–c). The OA plot of each sample shows SA behavior, as determined using the standard transmittance equation.37

3.6.1. 10
Fig. 7. Theoretically fitted Z-scan data of each sample: plots (a)–(c) show the open aperture mode and plots (d)–(f) show the closed aperture modes of GTO-1, GTO-2, and GTO-3, respectively. The solid curve illustrates the theoretical fitting of Z-scan data, while the dotted coloured spheres represent the measured data.

Fig. 7

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3.6.2. Closed aperture nonlinear refraction

The nonlinear refraction behavior of each GTO sample is studied using a closed-aperture (CA) Z-scan and plotted in Fig. 7(d–f). In the CA study, a negative n2 value suggests a peak followed by a valley (self-defocusing) curve, and a positive n2 value suggests a valley followed by a peak (self-focusing). The CA plots of each sample show a peak–valley pattern. All the samples have a negative n2, showing self-defocusing behavior. The observed nonlinear refraction is well-fitted using the standard CA Z-scan equation.38

3.6.2. 11

Here, Δφ is the phase shift due to nonlinearity.

n 2 is calculated using the equation given below.

3.6.2. 12

The wavelength used for measurement (λ) is 532 nm.

Here, both the OA and CA plots are shown in Fig. 7(a–f), showing good agreement between experimental data and theoretical fit, providing the reliability of measurement. The β values are 4.074 cm W−1, 3.173 cm W−1, and 3.085 cm W−1 for GTO-1, GTO-2, and GTO-3, respectively. The values of the linear refractive index are 2.114 cm2 W−1, 2.150 cm2 W−1, and 2.132 cm2 W−1, respectively. The real and imaginary parts of the third-order nonlinear susceptibility (χ(3)) are given by

3.6.2. 13
3.6.2. 14

Here ε0 is the permittivity of free space, and its value is 8.85 × 10−12 Fm−1. C is the velocity of light, and n0 is the linear refractive index of the sample. The real and imaginary parts of the nonlinear susceptibility depend on the n2 and β values, respectively. After determining the magnitudes of the real and imaginary parts of the third-order nonlinear susceptibility, the magnitude of the third-order nonlinear susceptibility is calculated using the formula mentioned below.

3.6.2. 15

The energy of the incident laser beam corresponding to a 532 nm laser source in the Z-scan technique is 2.33 eV. So, the incident photon will be unable to undergo a direct absorption process, as the determined value of Eg is 4.3 eV, 4.1 eV, and 4.2 eV for samples GTO-1, GTO-2, and GTO-3, respectively. However, the combined energy of two photons of 532 nm wavelength is 4.66 eV, which provides sufficient energy for absorption to occur. Since the criteria for two photon absorption are satisfied, i.e., 2.33 eV < Eg (4.1–4.3 eV) < 4.66 eV, the occurrence of two-photon absorption is observed in all the samples.

The obtained values of the NLO parameters and the figure of merit of each sample are mentioned in Table 2. The linear and nonlinear refractive indices of GTO-2 are higher compared to those of the other two samples. The observed values of the refractive index are in good agreement with the band gap relationship. The value of the real part of the third-order nonlinear susceptibility χ(3) is greater than the imaginary part for all the samples, as presented in Table 2. Among all the samples, the performance of GTO-2 is better. This indicates the dominant nature of the nonlinear refractive index over the nonlinear two-photon absorption. The NLO parameters of different materials operating using a CW laser are mentioned in Table S5.

Table 2. Estimated values of the NLO parameters and figures of merit of all samples.
Sample n 0 β( ×10−3 cm W−1) n 2 ( ×10−5 cm2 W−1) χ R (3) (×10−4) (esu) χ Im (3) (×10−4) (esu) χ (3) (×10−4) (esu) T FOM
GTO-1 2.114 4.074 2.181 24.729 1.955 24.806 0.009 100.628
GTO-2 2.150 3.173 3.305 38.779 1.576 38.811 0.005 195.789
GTO-3 2.132 3.085 1.760 20.298 1.506 20.353 0.009 107.237
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The nonlinear optical properties of various materials for all-optical switching applications are determined by a key parameter, the figure of merit. For an optical switching application, the criteria are Inline graphic, where T is the two-photon absorption figure of merit. The nonlinear figure of merit (FOM) is the inverse of T, and the necessary condition is Inline graphic. The value of T (<<1) and FOM satisfy the required conditions for all the samples. The value of FOM >10 is ideally suitable for all optical device applications.39–43 GTO-2 shows the maximum FOM value, i.e., 195.789, and the minimum T value, i.e., 0.005, among all the samples at a wavelength of 532 nm. That means the nonlinear refractive index dominates over two-photon absorption. Overall, the performance of all the samples is good, and GTO-2 is better. An ultra-low T value (T <<1) will be suitable for ideal switching, for next-generation optical device applications. It is crucial to remember that the Z-scan measurements were made with a CW laser, which may have contributed to the reported nonlinear response due to thermal effects. To separate electronic and thermal contributions and determine the true intrinsic third-order nonlinear coefficients, future research employing picosecond or femtosecond pulsed lasers would be beneficial.44

With increasing Te content from GTO-1 to GTO-2 to GTO-3 (precursor Gd : Te ratios of 1 : 1, 1 : 2, and 1 : 3), the average crystallite size increases (39.6, 48.6, and 64.8 nm), while the dislocation density and lattice strain decrease (Table S1), indicating improved crystallinity with fewer structural defects. The optical band gap first narrows from 4.3 eV (GTO-1) to 4.1 eV (GTO-2), consistent with Te-related defect/sub-band-gap states associated with the higher Te content, and then partially recovers to 4.2 eV in GTO-3, consistent with the improved crystallinity (larger crystallite size, lower strain) reducing the defect density. Correspondingly, GTO-1 (widest gap, fewest carriers) shows the lowest dark current and the highest photosensitivity (52.46%), while GTO-2 (narrowest gap) shows the largest photocurrent and highest responsivity (0.395 AW−1). The same narrow-bandgap, higher-defect-density GTO-2 sample also shows the largest nonlinear refractive index (3.305 × 10−5 cm2 W−1) and the highest figure of merit (195.789), indicating that Te-induced bandgap narrowing enhances the third-order nonlinear refraction. This Te-content–defect–band gap–photoresponse–NLO correlation has been added to provide a more complete structure–property picture of the GTO system.

4. Conclusion

In summary, GTO samples were synthesised by a hydrothermal method, which is easy and cost-effective. The surface morphology of all samples showed mostly a nanorod-like structure. A wide band gap between 4.1 eV and 4.3 eV was observed for all samples. The obtained band gap will be favourable for various optoelectronic applications. A rise in photocurrent was seen for all the samples. The observed fall time is more than the rise time, making the material prominent for high-speed device applications. Among all the samples, GTO-1 showed the best performance for photodetector applications. GTO-1 PD showed low responsivity, high detectivity, and high sensitivity among all the PDs. The third-order NLO properties of the GTO samples were observed using a single-beam Z-scan system with a diode-pumped CW laser. OA and CA data showed saturable absorption and two-photon absorption, respectively, for all samples. High FOM values of all samples indicate the suitability of the material for optical switching applications. The overall performance of GTO-2 is better than that of the others. The maximum FOM value of 195.789 and the minimum T value of 0.005 demonstrate the suitability of the GTO-2 nanomaterial for all nonlinear optical devices and optical switching applications.

Conflicts of interest

There are no conflicts to declare.

Supplementary Material

NA-008-D6NA00631K-s001

Acknowledgments

R.N. thanks the ICT-IOC Central Instrumentation Center for the different characterisation studies.

Data availability

All the data related to the present work are included in the manuscript.

Supplementary information (SI): schematic of hydrothermal synthesis, photo-response and Z-scan setup; structural parameters; elemental percentage; comparison of photodetection parameters; comparison of NLO parameters; EDX spectrum; IV plots on a linear scale. See DOI: https://doi.org/10.1039/d6na00631k.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

NA-008-D6NA00631K-s001
NA-008-D6NA00631K-s001.pdf (1.2MB, pdf)

Data Availability Statement

All the data related to the present work are included in the manuscript.

Supplementary information (SI): schematic of hydrothermal synthesis, photo-response and Z-scan setup; structural parameters; elemental percentage; comparison of photodetection parameters; comparison of NLO parameters; EDX spectrum; IV plots on a linear scale. See DOI: https://doi.org/10.1039/d6na00631k.

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