The paper shows the possibility of producing photosensitive structures using physical methods without the use of chemical synthesis and vacuum techniques. Suspensions of CdSe nanoparticles were produced by pulsed laser ablation in a liquid medium (chloroform), and layers of semiconductor material (CdSe) and conductive layers (carbon nanotubes) were formed at the gas-liquid interface and transferred to solid substrates by the Langmuir–Schaeffer method. The size of CdSe nanoparticles in the layers ranged from 60 nm to 200 nm, and the bandgap was 2.3 eV. Multilayer film structures consisting of a layer based on synthesized CdSe nanoparticles and a layer of conductive carbon nanotubes, that demonstrated photosensitivity in the optical region, were obtained. The value of current density under lighting at 1 V and –1 V equals 1.25 mA/cm² and –1.35 mA/cm², respectively. The maximum ratio of the illumination current density to the dark current value was 13.5 times (at a voltage of 1 V).

CdSe, conductive carbon nanotubes (CNTs), laser ablation, Langmuir–Blodgett technology, photodiode, semiconductor

Development of simpler and cheaper technological processes is one of the most important stages in the development cycle of any technology. This also applies to production techniques that can be used to obtain multilayer functional structures for photosensors and photoconversion devices. Technological processes that exclude the use of vacuum technology and involve only the most simple chemical processes that are less demanding on the purity of reagents are especially interesting for potential application.

Most methods can be classified as chemical or physical. Typical examples of methods of the first group are sol-gel synthesis, synthesis in micelles, chemical precipitation, etc. Such methods are difficult to implement, they often use expensive starting components and tools, in addition, the synthesis takes quite a long time. Physical methods (such as the molecular beam epitaxy method and other vacuum methods), as well as a group of physicochemical methods (which can include mechanochemical synthesis, solvothermal synthesis and synthesis in supercritical solvents) require special technological equipment and a combination of complex conditions – pressure, temperature, rate of deposition of materials on the substrate, duration of exposure to the material, creation of a special deposition/synthesis environment [1, 2].

Laser ablation is a well-known method that involves ejection of macroscopic amounts of materials from a surface of a solid usually induced by the interaction of short (~10^-13 to 10^-8 s) and intense (~10⁶ to 10¹⁴ W/cm²) laser pulses with the surface [3].

According to various sources [4, 5], the greatest advantage of laser ablation is the fact that this technique does not require any additives or precursors, thus, resulting in very high purity of the products of synthesis. In addition, laser ablation is capable of stoichiometric transfer of target material to the substrate (even for very complex materials) and is very versatile as the high-intensity focused pulsed laser can ionize a majority of materials to produce either single- or multielement compounds, multilayers, nanoparticles and nanostructures. It provides an opportunity to accurately control the growth rate and flexibility of various experimental parameters, i.e., ambient gas nature and pressure, gas flow rate, substrate temperature, laser incident intensity, number of applied pulses, pulse repetition rate, duration and wavelength, the substrate-to-target separation distance, target composition and structure and power density. It is capable of depositing multicomponent layers, shows an unlimited degree of freedom in the geometry of the experimental set-up, and is a clean and safe technique due to the use of light [5]. Laser ablation is often used to produce both thin films and nanosized objects of various composition [3, 6]. The composition of produced films mostly depends on the target composition. Thus, it is an attractive technology for the synthesis of nanoparticles (NPs) and composite materials, such as CdSe or similar semiconductor-doped glasses [7–9]. Another important factor affecting the nature of products is the chemical composition of medium where the laser ablation is performed. Pulsed laser ablation in a liquid (PLAL) has many specific features and advantages [3]. The liquid medium provides for a more effective heat dissipation from the impact zone of laser radiation. Swift supply of a large amount of energy results in formation of a very small size reaction zone. The overall dimensions of nanoparticles formed within this zone depends on the quantity of substance located there. This makes laser ablation quite similar to micelle-based NPs synthesis [10–13].

Abd A.N. et al. [14] described the method of producing colloidal spherical NPs of CdSe by laser ablation of cadmium selenide target in methanol and toluene solutions and showed that the size and shape of produced particles have a strong dependence on the liquid chosen as a medium for synthesis. Laser ablation in toluene produced well-defined CdSe NPs with an approximate diameter of 50 nm, while NPs ablated in methanol had an approximate diameter of 34 nm. In liquids, sedimentation of synthesis products to solid surfaces is complicated and can be viewed as a separate independent process.

Another promising physical method of production of nanosized film structures is Langmuir–Blodgett (LB) technique. There are many sources noting that this technique can become a suitable alternative to vacuum methods [15]. The key feature of this technique is the presence of amphiphilic properties of the molecules of substances forming a floating layer. These properties uniquely determine the position of molecules at the “gas–liquid” interphase and contribute to the formation of a monomolecular crystalline 2D-layer [15, 16].

Amphiphilic properties and low solubility in water determine the stability of floating layers and their technological characteristics. To form a floating layer from materials that do not have amphiphilic properties, it is necessary to integrate these substances into amphiphilic matrices and prepare nanocomposites with the required parameters. This way allows the formation nanofilms containing such large objects as conductive carbon nanotubes (CNTs) [17–19] or CdSe NPs [20–23].

Often, oleic acid or trioctylphosphine oxide are used as stabilizers and surfactant matrices for nanoparticles, and they are added at the NP synthesis stage. And, as a rule, the chemical synthesis of nanoparticles assumes the presence in the solution of an excess of surfactant that is not bound to the surface of the nanoparticles. But this is precisely the significant drawback of chemical synthesis methods. There is no way to accurately calculate the amount of free (i.e. not bound to the surface of the NP) surfactant in the solution. Therefore, an additional procedure to remove excess surfactant is often required. [24]. In this regard, laser ablation methods are a much “cleaner” alternative.

Returning to the LB method, one cannot ignore another important feature: it allows the application of conductive layers without creating short circuits. Of course, there are some difficulties and contradictory points here. Amphiphilic substances are usually dielectrics, but molecules of conducting materials do not have amphiphilic properties. To resolve these contradictions, composite compositions based on an amphiphilic matrix and a conductive component as inclusions are used, as mentioned above.

Such large nanosized objects as CNTs (if their surface has not been specifically modified) are hydrophobic, insoluble in water and at the same time are molecules that conduct electrical current well. This makes them a good choice for Langmuir layer formation. Of course, their complex structure and the lack of prominent amphiphilic properties makes it impossible to produce layers with high degree of order. But due to the chaotic interweaving of nanofibers, CNT layers have anisotropic structural and electrophysical properties. This allows avoiding additional problems associated with the manifestation of isotropy of electrophysical properties, which exists, for example, in sheet graphene. And the stability of the CNT layer can be increased by including in a surfactant matrix or additives that functionalize the CNT surface. Such functionalization leads to an increase in the solubility of CNTs in organic solvents, minimization of interactions between CNT molecules, and therefore to a more uniform distribution of molecules on the water surface and their greater order in the floating layer [17–19].

Floating layers of NPs or CNTs formed at the surfaces of a water subphase can be transferred to solid substrates using Langmuir–Blodgett or Langmuir–Schaeffer (LS) techniques (with horizontal or vertical substrate orientation during application, respectively). In this way, we can create CNT and NP films layer by layer, being able to control the individual composition and molecular organization of each layer at our discretion. [19, 22, 25, 26].

The following achievements can be noted as some examples demonstrating the capabilities of the LB method. High stability of the layers formed on the surface of water and the local order of CNTs in films on solid substrates can be directly caused by the modification of the CNT surface with crown ethers. This also allows for the successful transfer of Langmuir layers of multi-layer and single-layer CNTs onto hydrophilic silicon substrates [27, 28]. Jia et al. [29] showed that, in addition to functionalization of CNT surfaces with an amphiphilic compound, octadecylamines, the length of CNT also affects the structural orientation of CNTs in the film. The authors studied the CNT-based films and layers with length ranging from 500 to 3000 nm. They showed that the lower is the length of CNT the greater is their capacity for ordering and orientation in any particular direction. Fedoseeva et al. [30] showed the results of CdS NPs synthesis on the surface of vertically oriented CNTs. The X-ray photoelectron spectroscopic analysis showed that annealing and formation of a hybrid structure leads to the transfer of electron density from CdS to CNTs.

Thus, it can be seen that the interest in films based on NP and CNT is consistently high. This concerns both the preparation of solutions, the methods of formation and transfer of layers, and the influence of post-processing on the properties of these films. The aim of our research is to develop a technology for creating multilayer photosensitive structures based on CdSe nanoparticles and CNTs by combining PLAL with LS technique.

The initial reagents included CdSe powder with a particle size of 44–150 μm (CdSe medium, 99–99.999 wt.% purity, Hunan Fushel Technology Limited, China), multiwalled CNT (>95 wt.% purity; diameter ≈8–50 nm; length ~1–50 µm; ash <1.5 wt.%) acquired from CTI materials (GRAFTON Company, USA); chloroform with 99.95% purity (Component-Reactive, Russia), a square FTO (Fluorine-doped Tin Oxide) glass sample sheet with resistivity of ~10 Ω/sq. and an area of 2.25 cm² size and a square silicon (p type <111> orientation) sheet with an area of 0.25 cm² and resistivity 1–10 Ω·cm and thickness (400 µm).

The target was made by pressing 5 g of CdSe powder into a tablet with a diameter of 15 mm and a thickness of about 3 mm using a 5 t press. The tablet was placed to the bottom of a 25 ml glass beaker that was then filled with chloroform. The height of the chloroform layer above the tablet was close to 5 mm.

Irradiation was performed with a Nd:YAG laser at a energy of 700 mJ with wave length of 1064 nm, a pulse width of 10 ns, and a pulse train repetition rate set to 2 Hz. Laser ray (diameter – 1.2 mm) was focused from the source by a focusing lens onto a point on the target with approximate size 20 µm. The estimated radiation energy density at the focus was 0.22·10⁶ J/cm².

The suspension with CNTs was homogenized using the Hielscher Ultrasonics UP200Ht (Hielscher, Germany) handheld ultrasonic homogenizer operating at the output of 200W and frequency of 26 kHz.

The transmission and absorption spectra were registered with the Shimadzu UV-1800 spectrophotometer (Shimadzu, Japan). The photoluminescence spectra were recorded using the installation based on the Ocean Optics QE Pro-FL broadband spectrometer capable of recording PL spectra in the wavelength range of 380–1100 nm using the laser beam excitation at 410±10 nm KN2002 Langmuir–Blodgett (LB) troughs (KSV Nima, Finland) were used to transfer the layers to solid surfaces.

Characterization of surface morphology of samples with CdSe NPs and CNTs was performed with Mira II LMU Scanning Electron Microscope (SEM) (AztecLive Advanced Ultim Max 40 Inca Wave 500, TESCAN, Czech Republic) and Inspect F50 (Spain) Field Emission Scanning Electron Microscope (FESEM).

The samples reaction to lighting was studying by subjects the samples to Solar simulator LSS9000 IV characterization system (Fytronix Electronic Technologies, Turkey) as a white light source with density of luminosity ranging from 1 to 100 mW/cm². The current-voltage curves (CVC) were registered with Agilent (Keysight) B1500A parameter analyzing equipment and PM-5 probe station. The module of the parameter analyzing equipment allowed making precise measurements with current resolving power of 10 fA (if the power source voltage is up to 100 V).

Thus, we successfully produced photosensitive film structures consisting of CdSe NP layer (active layer) and CNT layer (counter electrode) obtained via Langmuir technique. The upper CNT layer had good conductivity – sufficient for a contact layer. At the same time, the application of the CNT layer did not lead to short-circuiting or any disruption of the structure of the NP layer located under the CNT layer (as is often the case when applying the upper contact by vacuum methods on single layers of nanoparticles with a thickness of several nanometers).

We successfully used PLAL to produce CdSe NPs for these structures and did not use any chemical synthesis techniques. Thus, we showed the possibility of producing nano-sized CdSe suspensions (with nanoparticle size ranging from 60 to 200 nm) in non-polar solvent without the need of a stabilizing agent.

The photocurrent density exceeds the dark current by 13.5 times at a voltage of 1 V. Thus, the obtained structures containing CdSe NPs and CNTs can be used in photoelectric devices, while elements of the presented production technique can be used to make potential photosensitive devices (such as photoresistors, photovoltaic equipment, and the like).

The study was supported by the Russian Science Foundation (project No. 21-73-20057) and Saratov State University, Russia.

Conflict of interest

The authors declare that they have no conflicts of interest.