Case study of electrical, structural and optical properties of CdSe Quantum Dots using DFT based computational tools.

Recent advancement in the field of nano-structured electronic devices has finely tuned up various progenition and manufacturing process of Quantum Dots that can directly harvest energy from solar power. Duly acknowledging different types of procuring processes, the paper primarily focuses on eight different CdSe (Cadmium Selenide) Quantum DOT samples which are initially synthesized in the material testing lab from the available precursor. Structural and electrical properties of these synthesized nano materials are inspected using DFT (Density function theory) based commercial tool - ESPRESSO. Later on, different photo luminous spectroscopic methods are used to understand the optical characteristics of the CdSe based nanomaterials. Light scattering method is applied to comprehend greater insights about the same sampled nano materials based on different parameters such as – polydispersity index (PI), Stokes shift analysis, zeta potential etc. Finally, the most equipped CdSe Quantum DOT sample is chosen for future purpose and a sensitive case study covering diverse advantages, disadvantages and future challenges of QDOTs is carried out.


Introduction
The materials that possess nanostructure are of immense interest due to their potential to bridge the chasm between the macroscopic and atomic scales, opening up entirely new application possibilities, particularly in electronics, optoelectronics, and biology [1]- [3].Apart from these industries, nanomaterials are also being extensively used in agricultural production, feed materials and in food additives to meet the ever increasing demand of human consumption [4].Nanotechnology has been centred on Quantum Dots, nanoscale semiconductor particles with exceptional optical, electrical, and photonic properties.This field of engineering is relatively new.Since their creation, quantum dots have been used and researched for a wide range of applications, including cancer detection, drug delivery, therapy, a replacement for organic dyes, and also as effective fluorescents because of their versatility in terms of size and applications.[5].QDs are zero-dimensional in comparison to the bulk, and the density of states (DOS) of QDs contains discrete quantized energies as a result of the limited number of electrons.Despite being zero-dimensional, quantum mechanics views it as a box; the size of the box is crucial.Sometimes a single electronic charge in a Qdot acts to repel the addition of another charge, causing an I-V curve DOS.The radius of the Qdots is reciprocal to the step size of the staircase.Due to 1291 (2023) 012032 IOP Publishing doi:10.1088/1757-899X/1291/1/012032 2 its optoelectronic and optical properties, we concentrate on the compound semiconductor-based nanostructured material CdSe and its multimodal applications in this lab.[6], [7].
In 1932, Rocksby used x-ray diffraction (XRD) to identify that the colours were caused by crystallites of CdS and CdSe [8].In 1982, Efros and Efros proposed that by altering the size or stoichiometry of CdSxSe1-x [9].Rosetti et al. explained the colour change of colloidal semiconductor solutions in 1991 [10].Several distinct methods of synthesis were developed during this time period [11]- [14].A new nano structured QD WO3 was synthesized by novel pulsed leaser ablation method and the morphological investigation was carried out in [15].Colloidal dispersion method was used to synthesize InP, a very narrowly sized quantum dot made from the precursor of chloroindium oxalate complex solution P(SiMe3)3 by Olga I and et al [16].Ethylene diamine tetra acetic acid (EDTA) was introduced as a facilitator to produce PbS quantum dot without any aggregation of Pb 2+ -solving an agelong problem in the field of integrating nano structured quantum dots [17], [18].
Density function theory (DFT), which was developed after the discovery of quantum mechanics in the latter 20th century, has emerged as a remarkable tool for investigating the optical, electrical, structural, and mechanical properties of nanostructured materials.Although the Schroedinger's equation solution for a single electron is the fundamental idea behind DFT-based tools, ongoing improvements and sophisticated approximations from numerous predecessors have made it reliable to investigate the aforementioned properties of any system [19], [20].A density function-based simulation tool is significant because it can foretell a variety of behaviours and properties of a system made up of parallel layers of different materials, even when the environment is constantly changing, and that too without the input of any experimental data.The fundamental principle of this DFT-based modelling is that, rather than focusing on a single electron, electron density of any system is taken into account, which can represent all of the system's ground state properties, including energy, diffusivity, etc. Numerous projects have used a customised DFT-based method up to this point [21], [22].Shiqi Zhou and coworkers have proposed a new use for this DFT-based simulation technique, in which he used classical density function theory to bridge the gap between uniform and non-uniform systems by demonstrating alignment of the first order and second order direct correlation functions.[23].By developing a thorough understanding between time-dependent and classical density function theorems, different luminescent properties, such as UV spectra, IR spectra, etc. are investigated for metal-organic framework (MOF-5) [24][25] [26].Svetlana V. Kilina and colleagues have proposed a novel modification of the fundamental electron-photon interaction to improve the efficiency of CdSe and PbSe QD based solar cells by generating multiple bottleneck relaxations between electronic and vibrational frequencies.[27].
Energy harvesting nanoparticles has greatly increased due to the desire to comprehend many fundamental Qdot properties and commercialization efforts.[28]- [30].In this lab, commercially available precursors in solution were used to create the robust and well-studied colloidal nanoscale material known as cadmium selenide quantum dots (CdSe QDs) under inert conditions at high temperatures.In this project, the structure-electrical properties of one of the well-known nanomaterials which has a hexagonal Crystal System, were studied using DFT-based software called Quantum ESPRESSO.Previous material science projects provided the background information on CdSe, and the outcomes of this project will aid in the computational modelling of devices based on quantum dots.To describe the optical characteristics of these nanocrystals, spectroscopy techniques such as UV-visible and photoluminescence were employed.These nanocrystals were cleaned up, and then dynamic light scattering techniques were used to characterise them.A quantum dot sample was finally made for use in applications that harvest and emit light.

Methodology
• Synthesis of CdSe nanoparticles for hydrogen evolution reaction I.
Selenium precursor was prepared using 60 mg of selenium powder; 4.0 mL of 1octadecene and 1.0 mL of Trioctylphosphine.II.
Cadmium precursor was prepared in a 25 mL three-neck flask using 134 mg of Cadmium oxide, 8 mL of 1-octadecene (90%), and 1.20 mL oleic acid.These mixtures were stirred in a vacuum environment for 5 minutes at 1500 rpm.III.
After 5 minutes in a vacuum, nitrogen flow was again started and the whole apparatus was wrapped in glass wool to insulate.

• Synthesis of CdSe quantum dots
i.
3 mL of toluene was taken in a set of 11 cultivation tubes.ii.
After cadmium precursor solution reached 225 o C, waited till solution starts to turn clear.iii.
5 mL of selenium precursor was rapidly injected.iv.
8 samples of aliquots (at the reaction time of 8s, 16 s, 32 s, 1 min 30 s, 2 min 30 s, 3 min, 4 min, 5 min ) were extracted over the course of the 5 min reaction.The final aliquot was taken when the temperature finally falls 150 o C.And the set of aliquots was observed with the UV-lamp.
Approximately 15-20 mL of CdSe in octadecene was carefully poured into a plastic 50 mL centrifuge tube when the reaction mixture reaches 100 o C. II.
7-10 mL of CdSe dots was transferred to a second centrifuge tube.And 15 mL of ethyl acetate were added to each centrifuge tube.Centrifuge tubes were filled to the 45 mL mark with methanol to precipitate the quantum dots.Following this, the tubes were centrifuged for 10 min at max RPM to crash out the particles.III.
The organic waste was Poured off and resuspended the quantum dots in about 3 mL of toluene.Following that, ~ 7.5 mL of ethyl acetate was added.The tubes were filled to 30ml with methanol and centrifuged to crash out particles.IV.
Repeated step 3 for 3 times.V.
Resuspend the particles in 10 mL of toluene and carefully pour each 10 mL sample of QDs into its own scintillation vial.
The collected samples were characterized using UV-Vis and DLS.

Results
Optical, electrical and structural properties of the synthesized CdSe QD are enlightened following two methods -Photoluminescence (PL) analysis method and Dynamic Light Scattering (DLS) set-up.A non-contact and non-destructive material probing system such as PL analysis captures the luminescence spectrum generated from CdSe QD as it falls to the ground state from excited state.To understand the structural properties and size variation of the QD in the solution, DLS method is followed and the outcomes are presented in terms of their comparative zeta potential and dispersity index.

Absorbance and photoluminescence (PL) analysis
Photoluminescence analysis is done to understand the opto-electrical properties of the integrated quantum dots.These characteristics are represented for eight different samples with respect to their invigilation time and in terms of their absorbance of light irradiated from a laser beam and emission upon the cessation of it.

Absorbance vs Wavelength plot
When a number of luminescent spectra are reflected off the solution, the quantum dot absorbance phenomenon occurs.The QD sample elevates to a higher energy level after absorbing the photon.The absorbance of the synthetic QD is examined eight times during this demonstration in order to fully grasp its absorbance potential.The absorbance of the quantum dot for various wavelengths at their corresponding times is shown in Fig. 1.Even though the overall absorbance of the QD initially rises with the passage of time, a general decaying trend of absorbance can be seen for any specific wavelength (between 550 nm and 625 nm) at its specific time.This merely asserts that the CdSe QD sample is better equipped to absorb energy at shorter wavelengths than it is at longer wavelengths.
According to Fig. 1, the wavelength (320 nm) exhibits a much higher absorption (4.5) when exposed to light for longer than 5 minutes.Initially, it exhibits an absorbance of about 0.50 for its initial 8 second exposure to light.It is easier to argue that the CdSe quantum dot exhibits higher absorption when its exposure time to light is increased because the other six time-variant samples drift similarly.On the other hand, for any particular time-dependent observed sample, the absorbance falls exponentially as the wavelength increases, even though it initially manifests a higher absorption value (apart from some absorbance peaks).For instance, a sample that was exposed to light for 4 minutes had a higher absorbance of 3.75 at the 320 nm initial wavelength compared to 0.75 at the 525 nm initial wavelength.This alone, however, does not change the fact that the timely observed quantum dot samples displayed absorption peaks occasionally as the wavelength increased, and that the number, frequency, and size of these absorbance peaks are sensitive to the length of time the samples are exposed to light.

Normalized emission spectra vs wavelength plot
The CdSe QD sample emits proton after being given energy and raised to an excited level, but only before returning to the ground state.The characteristics of this proton emission are shown in Fig. 2, and the intensity of the emission has been classified according to how much light the sample was exposed to.It's interesting to note that the emission peaks on the intensity curve of the samples that were examined at 1, 2, 3, 4, and 5 minutes are at a higher wavelength than those on the initial samples, which were collected at 8 and 16 seconds.The sample that was collected after the initial 32 seconds of exposure had a value of 700 au within the range of 500 nm to 525 nm wavelength, and the sample that was examined after 5 minutes of exposure had the lowest intensity of the emission.A potential aspect of the intensity vs. wavelength plot shows that it is either better to harvest energy from the synthesised QD sample with little or no manifestation to light for samples with lower wavelengths or with greater manifestation to light for samples with higher wavelengths.The normalised plot of Fig. 3, in which the intensity is normalised under the usual scale of maximum intensity (700 a.u), provides a clearer understanding of the CdSe QD sample's intensity.The bench mark for the earlier inferred time-wavelength-emission incident is provided by this plot.The later samples corresponded to their higher wavelengths, whereas the earlier samples managed to find the maximum intensity at relatively lesser wavelengths.

Absorbance maxima vs emission maxima plot
The correlation in Fig. 4 indicates that the emission wavelength and absorbance wavelength are very close to one another.In other words, a particle's energy release and absorption are almost equal, suggesting that less energy is lost during the vibrational relaxation phase.

The energy of the Stokes shift analysis
The Stokes shift predicts the spectral position of the maximum absorption and emission band, making it a crucial parameter for comprehending the molecular spectroscopic behaviour of QDs.The wavelength difference between the absorption and emission peak maxima for a given transition is known as the Stokes shift (SS) [25].The Stokes shift value represents the sample's relaxation time from the initial state to the excited state.The stokes shift of the manufactured CdSe quantum dot is presented in terms of wavenumber under the current study, and the value is discovered to be 2659.89 cm-1.The calculation was performed using the observed values of 14492.75cm-1 for emission and 17152.65 cm-1 for absorption.The wavelength in cm-1 is simply expressed as a wavenumber.Only the local maxima of the absorbance curve were taken into account in Fig 5 .When this curve was plotted, the data to the left of the local maxima was ignored.Following that, the trajectory of the absorbance curve was predicted using higher-order polynomial forecasting.The primary loss mechanism that significantly reduces the efficiency of power conversion is reabsorption of emitted photons.To put it another way, the photon that was released will be easily reabsorbed in the emission layer.The absorption and emission spectra do not overlap very much or at all, though, when the stock shift is greater.As a result, very few photons will be reabsorbed, leaving us with a large number of photons that can be used to generate power.Therefore, solar applications benefit from a greater stock shift.

Fig 5:
Stokes shift of the wavelength

Gaussian Curve and Jacobian Conversion Analysis
The Jacobian conversion process is a forerunner method to deal with any ambiguity during the data manipulation process, whereas the Gaussian curve is an excellent distributive method to understand the behaviour of real valued randomly and independently generated data.To fully comprehend the electrooptical phenomena of the synthesised CdSe QD, both of these approaches are used.

Determination of the FWHM of the Gaussian Curve
The width of the distribution at a level that is only half the maximum ordinate of the peak is known as the full width half maximum (FWHM).The duration of pulse waveforms is one phenomenon that is described by FWHM.The spectral width of optical communication sources and the resolution of spectrometers are examples of a signal from a baseline value to a higher or lower value, followed by a rapid return to the baseline value.It can be seen in Fig. 6 that the sample examined after 2.30 minutes and 1.30 minutes has the sudden peaks for the FWHM duration.The intensity of the quantum dot generated emissions is plotted in terms of 1000 times of its original e.V.

Jacobian conversion
Spectra are typically expressed in wavelength units, but expressing data as a function of energy provides a more comprehensive view of the physical universe.Nonetheless, the inverse relationship between the two variables poses difficulties and has occasionally led to peculiar conclusions, such as the notion that the human eye's spectral response complements the solar spectrum.To assure enlightening scientific research, it is essential that spectral data be presented accurately.Photoluminescence (PL) spectroscopy is a widely used experimental method for the study of photochemistry and photo physics.Quantitatively analysing PL intensity and spectral profiles requires contemplating more than intensity versus wavelength.Nanomaterials engineered for luminescent devices typically exhibit a broad spectral window with multiple emissive sources, resulting in emission over a broad spectrum of wavelengths.
Prior to quantitative analysis, it is essential to resolve such ambiguities in PL spectra, which are frequently observed in nanocrystals of semiconductors.In addition, in order to account for these ambiguities and wavelength conversions, signal values must be scaled by (h*c/E2) and the Jacobian conversion, depicted in Figure 7, is conducted.This conversion not only modifies the geometry of the line, but also accounts for the wavelength's direct effect on energy.Therefore, aliquots emitting in green rather than red would have the same FWHM in terms of wavelength but differ in energy.As indicated in the first clause of the Jacobian conversion, employing energy units such as eV provides a more comprehensive representation of the physical world.The study provides the relationship between the diameter of the CdSe quantum dot and the wavelength.[31].
Here the wavelength (λ) of the CdSe quantum dots at the final time point is 583.5nm.So, the diameter of the particles can be calculated to be 4.0375 nm.

Evaluate DLS data
The photon correlation spectroscopy (PCS), also referred to as the dynamic light scattering (DLS) method, is a crucial tool for comprehending how nanoparticles or macromolecules in solution diffuse.The size and shape of the molecules or samples are taken into consideration when calculating the hydrodynamic radii of the micro molecules after the diffusion coefficient has been determined.Under this subsection, the size-shape comparison of the examined sample of CdSe QDs and the DLS corresponded data are presented.

Zeta potential
Zeta potential is the name for the force that prevents negatively charged particles from interacting with one another.The amount of negative charge in a solution or the amount carried by particles can both be calculated using the zeta potential.These negatively charged particles can form micro flocks and eventually macro flocks by adding a charge neutralizer to overcome this repelling force.

Comparison of the size measurement of the quantum dot samples
The pre-ligand exchanged quantum dots' size, as determined by DLS data, is approximately 9.429 nm. which is larger than the size that we estimated using the reference's equation [26].Given that the DLS experiment was carried out three weeks after the quantum dots were created, the size difference could be the result of the quantum dots being combined between the two experiments.As an alternative, the higher-order term's neglection in the equation's standard error could be to blame for this variation.Furthermore, the size analysis of pre-ligand exchange quantum dots shows that there are roughly 30% of 9.429 nm-sized quantum dots in the solution.The standard deviation of the sample is 1.812 nm.This indicates that between 5.805nm and 13.053nm is the diameter range for about 95% of the particles.The graph of the ligand exchange size analysis has three peaks; the longest peak is at 4601 nm, the shortest peak is at 63.25 nm, and the two largest peaks are at 306.9 nm and 63.25 nm, respectively.In addition, according to %Number, 6.8% of nanoparticles have a diameter of 306.9 nm, 3% have a diameter of 4601 nm, and 8% have a diameter of 63.25 nm.The results of the post-Lingard size analysis show that the increase in particle size is caused by the presence of CdSe-S2-particles rather than by the presence of CdSe in the toluene solution before the Lingard exchange.

Discussion
Apart from the successful manufacturing of these highly potent QDs, an aftermath and deliberate impact of different parameters on the integration process are discussed in this paper.Also, the influence these synthesized nanostructured particles will have in near future compared to some heavy and large-scale energy extraction devices is another motivation leveraged under this section.These potentialities of the newly synthesized CdSe QDs are journaled in three different sections to enlighten the reader more persuasively and discretely.

Fig 6 :
Fig 6: Fitting the PL spectrum to the Gaussian Curve.

9 Fig 7 :
Fig 7: Jacobian converted and uncorrected absorption and fluorescence spectra for CdSe nanocrystals

Fig 9 :
Fig 9: Size distribution by %Number -Pre Lingard size distribution by %Number.