# Functional Energy Accumulation, Photo- and Magnetosensitive Hybridity in the GaSe-Based Hierarchical Structures

^{1}

^{2}

^{3}

^{*}

## Abstract

**:**

_{2})

_{2}), GaSe(SmCl

_{3}) and GaSe(CS(NH

_{2})

_{2}(SmCl

_{3})) synthesized by an intercalation method. The conductive properties of synthesized clathrates and their relation to hierarchical structural complexity were explored by an impedance spectroscopy technique. The impedance response, thermostimulated discharge spectra, and photo- and magnetoresistive effects are reported. Based on the obtained results, the impurity energy spectra were calculated. A strong low-frequency inductive response, observable in the GaSe(SmCl

_{3}) clathrate, makes this material promising for the development of gyrator-free nanodelay lines potentially applicable in nanoelectronics. Hierarchical GaSe(CS(NH

_{2})

_{2}(SmCl

_{3})) clathrate, on the other hand, reveals hysteresis of the current–voltage characteristics, apparently confirming an accumulation of electric energy at interphase boundaries. A relevant spin battery effect, observable experimentally in stationary magnetic fields, demonstrates a principal possibility of the electric energy accumulation at a quantum level.

## 1. Introduction

_{2})

_{2}), GaSe(SmCl

_{3}), and GaSe(CS(NH

_{2})

_{2}(SmCl

_{3})), synthesised by an intercalation method. Conductive properties of the synthesised clathrates are characterized by impedance spectroscopy, aiming to explore the complexity of their hierarchical structure. The hierarchical structures exhibit the spin battery effect, demonstrating a principal possibility of the electric energy accumulation on a quantum level.

## 2. Materials and Methods

_{2})

_{2}), GaSe(SmCl

_{3}) and GaSe(CS(NH

_{2})

_{2}(SmCl

_{3})) were synthesized by the intercalation method. The photosensible quasi two-dimensional semiconducting crystals of gallium selenide (GaSe), grown by the Bridgman–Stockbarger method, were used as the subhost crystal material. GaSe belongs to the n-type semiconductors, with the band gap of 2.02 eV [18]. It is characterized by a well-developed layered structure [17] and the affordability of guest positions within Van der Waals regions oriented perpendicularly to the crystallographic c-axis (Figure 1). Due to a weak interlayer bonding, the interlayer space can be enlarged multiple times by applying the intercalation method. Such an expanded GaSe lattice appears to be appropriate for the intercalation of guest atoms or molecules [19,20].

_{2})

_{2}is one of the simplest thioamides (Figure 2) that was used as a host substance for the hierarchical clathrate formation. Because of its nonlinear optical properties, thiourea is widely applied in the fabrication of optoelectronic components and devices, such as polarizing filters, electronic optical switches, electrooptical and electroacoustic modulators and deflectors. Thiourea, on the other hand, is also widely used in different electrochemical processes [21]. The molecules of thiourea can form a matrix with unilateral disjointed channels of rhombohedral structure (see Figure 3) that are available for guest components, which are oriented in an unordered way.

^{−30}C⋅m and the dielectric constant is 2.224 [23].

_{2}melt at 300 °C for 5 min, resulting in a 4-fold lattice expansion. At the second stage, the NaNO

_{2}was deintercalated from the GaSe by washing the samples in deionised water (DW) until the pH value of the DW was reached. The samples were then dried in a vacuum at 110 °C, until their constant mass was achieved. Finally, at the third stage, the guest components were intercalated into the expanded GaSe matrix by a direct exposure of GaSe in relevant saturated solutions of CS(NH

_{2})

_{2}, SmCl

_{3}or CS(NH

_{2})

_{2}+ SmCl

_{3}, respectively, for 48 h at room temperature. The samples were then washed with DW and dried in a vacuum until the constant mass was reached. The intercalated GaSe-structures obtained in this way were subjected to X-ray diffraction (XRD) analysis carried out with a DRON-3 diffractometer, employing Cu-Kα radiation and an LiF single crystal as a monochromator.

^{−3}÷ 10

^{6}Hz. A Dirichlet filter [27,28] was applied to eliminate questionable datapoints. The frequency-dependant complex impedance Z was analysed with a graph–analytic method by means of a ZView 2.3 software (Scribner Associates, Inc., Southern Pines, NC, USA) package, and errors in approximations did not exceed 4%. The measurements were executed under normal conditions (NC) in an external magnetic field (MF) (strength value H = 2.75 kOe) applied perpendicularly to the sample, which was illuminated (L) by visible light (radiant flux 65 W).

## 3. Results and Discussion

_{2})

_{2}(SmCl

_{3})) structure is characterized by two evident maxima, suggesting a coexistence of regions with slightly different interlayer distances.

_{2})

_{2}into 4-fold expanded GaSe lattice leads to about 4-fold decrease in the real part of complex impedance ReZ of GaSe〈CS(NH

_{2})

_{2}〉 supramolecular ensemble and it is evident from relevant frequency dependences ReZ(ω), which are demonstrated in Figure 6. The frequency independent part of ReZ(ω) is observable for the non-intercalated GaSe lattice in the 10

^{−3}−200 Hz frequency range. The GaSe(CS(NH

_{2})

_{2}) supramolecular structure, however, reveals a stable ReZ magnitude in the entire frequency range.

^{3+}into a 4-fold expanded GaSe lattice results in a decrease in the ReZ value in the low-frequency region, by about one order of magnitude. The character of the ReZ(ω)-dependence for the GaSe(SmCl

_{3}) changes significantly at higher frequencies, where it falls substantially.

_{2})

_{2}(SmCl

_{3}) into the GaSe lattice leads just to a slight increase in the ReZ value compared to the GaSe(CS(NH

_{2})

_{2}) clathrate. Comparing, on the other hand, ReZ(ω)-dependences of the GaSe(CS(NH

_{2})

_{2}(SmCl

_{3})) and GaSe(SmCl

_{3}) clathrates, one may recognize their similarity only in the low-frequency region, roughly below 200 Hz. Hence, there is a non-additive influence of the subhost GaSe lattice, and the guest molecules in the frequency region related with a band carrier’s conductivity. It follows that there is a substantial influence of hierarchical modulating potential on the impurity electron spectrum, which provides current flow at room temperature. The density of states at the Fermi level N(E

_{F}), calculated with Pollak–Geballe theory [30] (see Figure 7), confirms the dominance of the impurity electron spectrum over the possible charge mobility growing perpendicularly to layers of the semiconductor. It is obvious that the actual density of deep trapping centres in the GaSe(CS(NH

_{2})

_{2}(SmCl

_{3})) is considerably higher compared to the associated influence of the guest CS(NH

_{2})

_{2}and SmCl

_{3}molecules. At the same time, histograms explicitly indicate a decrease in the hopping radius of charge carriers caused by the influence of the hierarchy of complex clathrate barrier potential.

_{3}) clathrate (curve 3) in Figure 6 are well described by Jonscher‘s formula [31,32] for resistivity ρ

_{0}is the limiting zero frequency conductivity, A is the pre-exponential constant, ω is the angular frequency, and s is the power law exponent, where 0 < s < 1. The second term in the denominator represents the hopping conduction. The first term represents the simple drift motion of electrons in a material, and is defined by the distribution function of the waiting period φ(t) describing the probability for an electron jump expected in the period of time t after the previous jump

_{2})

_{2}) (curve 2, Figure 6) and GaSe(CS(NH

_{2})

_{2}(SmCl

_{3})) (curve 4, Figure 6), on the other hand, are practically frequency-independent apparently, indicated by the hopping conduction mechanism realized with a sequence of identical potential wells without long-range interactions. Such an approach results in a unique value of activation energy, thus, the distribution function of the waiting period gets an exponential character. De facto, it is an implication of the hierarchical potential explaining the increase in a number of deep trapping centres. Therefore, these trapping centres can be modelled by a sequence of δ-wells. Considering this case, and when the Fermi level is far from the maximum density of states (${E}_{F}>>kT$), the conductivity σ reads as

_{0}is the radius of wave-function localization, E

_{a}is the activating energy, T is the temperature, k is the Boltzmann constant, and ħ is the Plank constant. Ignoring long-range interactions, the wave-function for the system of two δ-wells separated by distance r takes the form

_{0}is a solution of the equation

_{2})

_{2}), GaSe(SmCl

_{3}) and GaSe(SC(NH

_{2})

_{2}(SmCl

_{3})) structures are presented in Figure 8. The photoresistance value of the synthesised structures depends on the clathrates’ structural complexity. This fact correlates well with theoretical calculations of the impurity energy spectrum: the highest increase in the density of deep trapping centres is exhibited by the clathrate with the most complex hierarchical architecture (Figure 7). At the same time, the magnetoresistivity is remarkably less for GaSe(CS(NH

_{2})

_{2}(SmCl

_{3})) compared to SmCl

_{3}in the potential field of the subhost lattice only (Figure 8), in accordance with a decrease in the density of deep trapping centres in the magnetic field caused by a Zeeman modification of the impurity energy spectrum (Figure 9).

_{2})

_{2}, SmCl

_{3}or CS(NH

_{2})

_{2}(SmCl

_{3}) components changes the character of the complex impedance substantially (Figure 11). In the case of GaSe(SmCl

_{3}), the strong inductive response was observed in the low-frequency region. Such inductive response, however, disappeared in an applied external magnetic field. This phenomenon, known as negative capacitance [34,35], was studied intensively in a number of works [17,25,36], basically due to the potential related nanoelectronic applications—particularly, with the development of gyrator-free nanodelay lines. The equivalent electric circuit for the relevant low-frequency region of the Nyquist plot is presented in Figure 12. Using the complex amplitude method, such a circuit can be represented by analytical expression

_{I}and Z

_{II}, of Equation (14), as follows:

_{3}intercalation. At the same time, a strong charge carriers’ delocalization (ReZ >> ImZ) at Zeeman levels appears in the same frequency region.

_{2})

_{2}(SmCl

_{3})) demonstrates the combination of impedance contributions originating from the guest CS(NH

_{2})

_{2}(SmCl

_{3}) component and non-intercalated GaSe lattice (Figure 11). In addition, the impedance response measured in the magnetic field demonstrates an increase in the low-frequency range and is almost constant in its value in the high-frequency range, thus confirming the charge accumulation at the phase-interface region. Apparently, it is caused by Zeeman changes in the asymmetry of the density states above and below the Fermi level, which blocks resonant tunnelling.

- The light illumination of the GaSe(CS(NH
_{2})_{2}(SmCl_{3})) structure causes tunnel current flow, levelling the contribution caused by charge accumulation in the infra-low-frequency region (Figure 13c).

_{2})

_{2}), GaSe(SmCl

_{3}) and GaSe(SC(NH

_{2})

_{2}(SmCl

_{3})) structures. The hierarchy of relevant clathrate structures evidently influences the origin of the current spectra. Particularly, going from clathrate structures of low hierarchy, such as GaSe(SmCl

_{3}), to the clathrate structures of higher hierarchy, such as GaSe(SC(NH

_{2})

_{2}(SmCl

_{3})), it transforms from the band-type spectrum to the continuous one with homo- and heterocharge relaxations. The clathrate of complex hierarchical architecture, GaSe(CS(NH

_{2})

_{2}(SmCl

_{3})), appears to be promising material for a quantum accumulation of the electric energy. It particularly results from cycling voltammetry experiments. The hysteresis character of the current-voltage (CV) characteristics measured perpendicularly to the layers of the hierarchical GaSe(CS(NH

_{2})

_{2}(SmCl

_{3})) structure (Figure 15), confirm the assumption regarding the charge accumulation at the interphase boundaries. One should emphasize that the hysteresis behaviour remains practically the same in the case of a constant magnetic field and light illumination. Amazingly, in a zero magnetic field (H = 0), the light illumination alone does not result in a current flow. In combination with an applied magnetic field (H ≠ 0), however, the current is generated, as evidenced by the CV-characteristics presented in Figure 15b. Particularly, at the magnetic field of H = 2.75 kOe, the CV-curve intersects the voltage axis at 21 and −23 mV in forward and reverse directions, respectively. This fact confirms the appearance of a spin electromotive force (EMF), which may be considered a basic effect, lying in the fundamentals of quantum energy accumulation and the prospective spin capacitor devices and relevant applications [37]. The distinctive feature of this finding is demonstration of the spin EMF at room temperature and relatively weak magnetic fields.

_{2})

_{2}(SmCl

_{3})) structure. The anomalous increase of ReZ with temperature is observed in the temperature range 0–20 °C. To clarify the nature of such an effect, the temperature dependence of the density of states at the Fermi level N(E

_{F}), hopping radius R, trapping centres scattering J, and the density of the deep trapping centres are presented in Figure 17. The critical temperature point T = 20 °C represents the minimal distribution in trapping centres at a relatively low value of the density of states at the Fermi level. One must mention that a considerable impact on the carriers’ mobility perpendicularly to the nanolayers has scattering mechanisms being temperature-dependent.

_{2})

_{2}(SmCl

_{3}))—and the unique character of its thermostimulated depolarisation current spectra (see Figure 14d) and relevant quantum effects—suggest more detailed investigations of their electrical conduction properties that could shed light on the origin of the physical effects observed there. The magnetoresistive and photoresistive characteristics of the GaSe-based hierarchical structures, as well as prospective fields of their applications, are summarized in Table 1. The GaSe(SmCl

_{3}) structure evidently demonstrates quite a large magnetoresistive efficiency, opening prospects for magnetic sensing applications. The hierarchical GaSe(SC(NH

_{2})

_{2}(SmCl

_{3})) structure, on the other hand, exhibits abilities for the energy accumulation at quantum level. The spin battery effect revealed in this system opens prospects for magnetovoltaic applications.

## 4. Conclusions

- The complex hierarchical structures of GaSe(CS(NH
_{2})_{2}), GaSe(SmCl_{3}), and GaSe(CS(NH_{2})_{2}(SmCl_{3})) were synthesized by intercalation technique. The non-additive influence of both the subhost and the guest components on the real part of the complex impedance for the GaSe(CS(NH_{2})_{2}(SmCl_{3})) clathrate is observable in the frequency region characteristic for band carriers. This proves that hierarchical modulating potential has a considerable impact on the electron impurity spectrum, which provides current flow under normal conditions; - The GaSe(CS(NH
_{2})_{2}(SmCl_{3})) clathrate is characterised by an unproportionally higher density of deep trap centres compared to the associative influence of the guest CS(NH_{2})_{2}and SmCl_{3}components. The hierarchy of barrier potential, on the other hand, results in a decrease in the hopping radius of charge carriers; - The increase in the photoresistive effect is provided by the complexity of the hierarchical structure. The magnetoresistive effect in GaSe(CS(NH
_{2})_{2}(SmCl_{3})), on the other hand, is considerably smaller compared to SmCl_{3}in the subhost potential field. The Zeeman effect influences the impurity energy spectrum, thus decreasing the density of deep trapping centres in the magnetic field; - The strong inductive response, observable in the low-frequency region for the hierarchical GaSe(SmCl
_{3}) clathrate, makes this material promising for the development of gyrator-free nanodelay lines, with prospects for nanoelectronics applications; - The hysteresis in the volt-ampere characteristics of the hierarchical GaSe(CS(NH
_{2})_{2}(SmCl_{3})) clathrate confirms the electric energy accumulation at the interphase boundaries. The EMF observed in the experiment is generated due to the spin battery effect in the stationary magnetic field.

## Author Contributions

## Funding

## Conflicts of Interest

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**Figure 3.**Plane schematic representation of rhombohedral thiourea structure. Nine complete tunnels with a distance of 9.2 Å between their centres are shown.

**Figure 6.**Frequency dependences of the real part of the complex impedance of non-intercalated 4-fold expanded GaSe matrix (1 black) and synthesized GaSe(CS(NH

_{2})

_{2}) (2 red), GaSe(SmCl

_{3}) (3 green) and GaSe(CS(NH

_{2})

_{2}(SmCl

_{3})) (4 blue) structures measured along the crystallographic C axis.

**Figure 7.**Histograms of changes in the density of states at Fermi level N(E

_{F}) (

**a**), trapping centres scattering J (

**b**), hopping radius R (

**c**) and density of deep trapping centres N

_{t}(

**d**) relative to initial fourfold expanded GaSe matrix for GaSe(SC(NH

_{2})

_{2}) (1), GaSe(SmCl

_{3}) (2) and GaSe(SC(NH

_{2})

_{2}(SmCl

_{3})) (3).

**Figure 8.**Photo-(

**a**) and magnetoresistive (

**b**) effects measured in the low-frequency region for GaSe(SC(NH

_{2})

_{2}) (1), GaSe(SmCl

_{3}) (2) and GaSe(SC(NH

_{2})

_{2}(SmCl

_{3})) (3).

**Figure 9.**Histograms of change in the density of the states at the Fermi level N(E

_{F}) (

**a**), trapping centres scattering J (

**b**), and hopping radius R (

**c**) and density of deep trapping centres N

_{t}(

**d**) in applied magnetic field for GaSe(SC(NH

_{2})

_{2}) (1), GaSe(SmCl

_{3}) (2) and GaSe(SC(NH

_{2})

_{2}(SmCl

_{3})) (3).

**Figure 11.**Nyquist plots for the synthesised hybrid structures GaSe(SC(NH

_{2})

_{2}) (

**a**), GaSe(SmCl

_{3}) (

**b**) and GaSe(SC(NH

_{2})

_{2}(SmCl

_{3})) (

**c**) measured under normal conditions (black) and under applied magnetic field (red).

**Figure 12.**Equivalent electric circuit for the impedance response of GaSe(SmCl

_{3}) (Figure 10b, black curve) measured in low-frequency region.

**Figure 13.**Nyquist plots for the synthesised hybrid structures GaSe(SC(NH

_{2})

_{2}) (

**a**), GaSe(SmCl

_{3}) (

**b**) and GaSe(SC(NH

_{2})

_{2}(SmCl

_{3})) (

**c**) measured perpendicularly to layers under illumination.

**Figure 14.**Current spectra of thermostimulated depolarisation measured perpendicularly to layers of the non-intercalated 4-fold expanded GaSe matrix (

**a**), synthesized hierarchal GaSe(SC(NH

_{2})

_{2}) (

**b**) GaSe(SmCl

_{3}) (

**c**) and GaSe(SC(NH

_{2})

_{2}(SmCl

_{3})) (

**d**) structures.

**Figure 15.**Current-voltage (CV) characteristics of GaSe(SC(NH

_{2})

_{2}(SmCl

_{3})) measured perpendicularly to the layers under normal conditions (black), in the applied magnetic field (red) and under light illumination (green) (

**a**). Spin battery effect: experiment under normal conditions (black), in the magnetic field H = 2.75 kOe (red) and under light illumination (green) (

**b**).

**Figure 16.**Temperature dependence of the real part of the complex impedance for GaSe(SC(NH

_{2})

_{2}(SmCl

_{3})), measured perpendicularly to the layers.

**Figure 17.**Temperature dependences of the density of states at the Fermi level N(E

_{F}) and trapping centres scattering J (

**a**), density of deep trapping centres N

_{t}and hopping radius R (

**b**) for GaSe(SC(NH

_{2})

_{2}(SmCl

_{3})).

**Table 1.**The magnetoresistive and photoresistive characteristics of the GaSe-based hierarchical structures and the prospective fields of their applications.

Structures | Magnetoresistive Effect ln(ReZ _{MF}/ReZ_{0}) | Photoresistive Effect ln(ReZ_{L}/ReZ_{D}) | Inductive Response | Energy Accumulation at Quantum Level | Spin Battery Effect |

GaSe(CS(NH_{2})_{2}) | 0.09531 | −0.35667 | − | − | − |

GaSe(SmCl_{3}) | −1.60944 | −0.10536 | + | − | − |

GaSe(CS(NH_{2})_{2}(SmCl_{3})) | −0.22314 | −0.69315 | − | + | + |

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**MDPI and ACS Style**

Chabecki, P.; Całus, D.; Ivashchyshyn, F.; Pidluzhna, A.; Hryhorchak, O.; Bordun, I.; Makarchuk, O.; Kityk, A.V.
Functional Energy Accumulation, Photo- and Magnetosensitive Hybridity in the GaSe-Based Hierarchical Structures. *Energies* **2020**, *13*, 4321.
https://doi.org/10.3390/en13174321

**AMA Style**

Chabecki P, Całus D, Ivashchyshyn F, Pidluzhna A, Hryhorchak O, Bordun I, Makarchuk O, Kityk AV.
Functional Energy Accumulation, Photo- and Magnetosensitive Hybridity in the GaSe-Based Hierarchical Structures. *Energies*. 2020; 13(17):4321.
https://doi.org/10.3390/en13174321

**Chicago/Turabian Style**

Chabecki, Piotr, Dariusz Całus, Fedir Ivashchyshyn, Anna Pidluzhna, Orest Hryhorchak, Ihor Bordun, Oleksandr Makarchuk, and Andriy V. Kityk.
2020. "Functional Energy Accumulation, Photo- and Magnetosensitive Hybridity in the GaSe-Based Hierarchical Structures" *Energies* 13, no. 17: 4321.
https://doi.org/10.3390/en13174321