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Influence of transition metal alloying and surface modification of the CdTe quantum dots on their optical properties, band structure and electrochemical activity
Accepted Manuscript Full Length Article Influence of transition metal alloy in gand surface modification of the CdTe quantum dots on their optical properties, band structure and electrochemical activity Olena Tynkevych, Yuriy Khalavka PII: DOI: Reference:
S0169-4332(18)30998-X https://doi.org/10.1016/j.apsusc.2018.04.044 APSUSC 39043
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
19 September 2017 11 March 2018 5 April 2018
Please cite this article as: O. Tynkevych, Y. Khalavka, Influence of transition metal alloy in gand surface modification of the CdTe quantum dots on their optical properties, band structure and electrochemical activity, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.04.044
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of transition metal alloy in gand surface modification of the CdTe quantum dots on their optical properties, band structure and electrochemical activity OlenaTynkevych, Yuriy Khalavka Department of Solid State Inorganic Chemistry and Nanomaterials, Yuriy Fedkovych Chernivtsi National University, Kotsiubynsky Str. 2, 58012, Chernivtsi, Ukraine Abstract We report a study of CdTe quantum dots modification bythe alloying of with Zn2+ and Hg2+ions and shell growth. Optical and cyclic voltammetry measurements were used to explain changes in the band structure of the particles via shell growth and during the alloying. Influence of stabilizer on the electrochemical behavior of the particles is also discussed. Obtained nanostructures demonstrate photoluminescence in the wider range of wavelengths than pure CdTe QDs. Keywords: CdTe, quantum dots, transition metals, alloying, cyclic voltammetry, band structure. 1.
Introduction
CdTe quantum dots (QDs) due to their unique size- and composition-dependent optical and electrical properties are promising materials for different applications such as chemical sensors, solar elements, LED devices, in vivo and in vitro biosensing imaging etc. In addition to the widely known size- dependent optical properties of CdTe QDs there is an alternative method for tuning QDs optical and electrical properties by varying the QDs composition. Such doped/alloyed CdTe QDs have been produced via composition-dependent transition metals doping/alloying processes. The modifications of the optical and electrical properties of CdTe QDs can be achieved using different techniques. For instance, Chu and Van Hung [1, 2] demonstrated that optical and electrical properties of QDs can be changing by adjusted synthesis conditions. The incorporation of transition metals impurities into QDs is another useful technique used for tuning their optical properties [3, 4] due to the band structure changes. Also as shown by Doskaliuk [5] and Wang [6], prolonged heat treatment of the CdTe colloidal solutions leads to the significant shift of PL maximum to the long-wave region due to the CdS shell growth and the formation of type II core/shell CdTe/ CdS QDs. For a wide range of applications based on CdTe QDs, it is important to understand the effects of synthesis conditions, impurities incorporation and CdS shell growth on the band structure changes. Analysis of Cyclic voltammetry (CV) data of oxidation and reduction reactions involving CdTe QDs enables calculation of the position of CdTe QDs bandgap energy levels and various impurities levels position [7]. In the present work, we summarize application of cyclic voltammetry in the monitoring of band structure changes of CdTe QDs during transition metal alloying and the shell growth. 2.
Material and methods
The absorption and photoluminescence spectra were recorded at room temperature with OceanOptics USB-2000 spectrophotometer. The CV measurements were performed using a computer-controlled Potentiostat/Galvanostat PI-50-1. A three-electrode system with a glassycarbon counter electrode, an Ag/AgCl reference electrode and the platinum electrode as the working electrode was used for the cyclic voltammetry measurements. The cyclic voltammograms were obtained by scanning the potential from – 1.5 to 1.5 V at a scan rate of 100 mV/s. The elemental analysis was carried out on C115M1 Atomic emission spectroscope (AES). 2.1.
Synthesis of QDs
CdTe QDs stabilized by thioglycolic acid (TGA) were synthesized at room temperature in alkali aqueous solution by the reaction of CdSO4 and electrochemically generated H2Te in the presence of thioglycolic acid. The oxygen was removed from the reaction system by bubbling
argon through solutions for 15-20 min. After the reaction, the thermal processing of colloids at 100°C was carried out. Synthesis of Cd(Hg)Te, Cd(Zn)Te alloyed QDs was carried out by addition of different amounts Hg(NO3)2 and ZnSO4 respectively to the freshly prepared CdTe nanocluster colloidal solutions with subsequent heat treatment at 100°C. The chemicals and reagents used in this study: analytical grade 3CdSO4×8H2O of ≥ 99.0 % purity; thioglycolic acid of ≥ 98 % purity (catalog number T3758); NaOH of ≥ 99 purity (S8045); tellurium granular 99.99 % trace metals basis (263303) all from Sigma-Aldrich and analytical grade Hg(NO3)2 and ZnSO4 of ≥ 98% purity. 3.
Results and Discussion
3.1.
CdTe QDs band structure changes during the Hg-alloying.
The TGA-stabilized Cd(Hg)Te QDs were obtained by the addition of Hg2+ ions precursor into the freshly prepared colloidal solution of CdTe nanoclusters and subsequent thermal treatment for 5 hours at 100°C. The amount of added Hg2+ ions was 2% (CdHg(2)Te QDs) and 4% (CdHg(4)Te QDs) of Cd2+ content in the precursor solution. Such alloying leads to the formation of Cd(Hg)Te alloyed QDs by ion-exchange reaction [9, 10]. As a result, the bathochromic shift of PL peak from 588 nm to the near-infrared regions of the spectrum (730 nm) was observed (Fig. 1 A). The appearance of the additional PL peak at 910 nm can be explained by the formation of HgTe shell on top of QDs surface proving complete replacement of surface Cd2+ ions by Hg2+ ions [9]. The absorption maximum is shifted towards longer wavelengths and becomes less pronounced. Such optical properties caused by the changes in the band structure during the transformation of CdTe QDs into Cd(Hg)Te alloyed QDs by ion-exchange reaction [9, 10]. Most likely, the ion-exchange process occurs at the two stages: alloying of CdTe nanoclusters and further alloying of growing QDs during heat treatment. Fig. 1 B shows cyclic voltammograms recorded for a CdTe QDs colloidal solution (black line). The anodic peak at 1.28 V (marked as A1) and cathodic peak at −0.87 V (marked as C) are observed. The calculations of edge positions of the valence band (VB), conduction band (CB) and bandgap values were performed according to the approach described in [8]. Calculated bandgap value of 2.15 eV is in good agreement with the optical band gap obtained from the absorption peak maximum (2.20 eV). At the cyclic voltammograms for the CdHg(2)Te alloyed QDs colloidal solution (Fig. 1 B, blue line) the same oxidation peak at 1.28V, reduction peak at -0.87V and appearance of additional oxidation peak at 0.86 V (marked as A2) is observed. These changes can be explained by the formation of Hg energy levels in the band gap of Cd(Hg)Te alloyed QDs. It is important to note that in the case of CdHg(4)Te QDs (red line), the oxidation peak A1 at 1.28V has completely disappeared. Such effect may identify that many of cadmium surface atoms are replaced by mercury that leads to the formation of the core/shell structure CdTe/Cd(Hg)Te. A similar effect was reported in our previous work [9] for Cd(Hg)Te alloyed QDs. Based on the obtained CV data and calculations we plotted energy levels diagram of CdTe and Cd(Hg)Te alloyed QDs (Fig. 1B. Inset). Place for Figure1 To understand the effect of the stabilizing ligand concentration on the electrochemical activity of TGA-stabilized CdTe and Cd(Hg)Te alloyed QDs have been synthesized according to previously described methods with 100% excess of TGA. The amount of added Hg2+ ions was 5% of Cd2+ content in the solution. As shown in Fig. 2A, the absorption peaks at 550 nm can be seen only for first sample (CdTe QDs). Upon the addition of Hg2+ ions the absorption maximum was disappeared which is typical for Hg-alloyed CdTe QDs [10]. We also observed the bathochromic shift of the PL peaks from 586 nm (for the CdTe QDs) to the infrared region of the spectrum and occurrence of doublepeaks in PL spectra with maxima at 838 and 907 nm (for the Cd(Hg)Te alloyed QDs). This effect can be explained by the formation of HgTe shell on top of CdTe QDs surface as the result of the cation exchange process [9]. Nevertheless, as shown by the CV measurements, the electrochemical signal which corresponds to the electrochemical activity of QDs was not
detected. At the cyclic voltammograms for both samples, only the oxidation peak at 1 V and reduction peak at -0.9 V were observed (Fig. 2B). The observed signal is similar as for the 20 mg/L solution of TGA. The oxidation peak in the range around 1.0-1.3 V attributed to the electrooxidation of the thiol group of TGA [11-13]. This result indicates that excess of TGA leads to the formation of a dense stabilizing ligand layer on top of QDs which hinder detection of the electrochemical response of QDs (Fig. 2B Inset). Place for Figure2 3.2.
CdTe QDs band structure changes during the Zn-alloying
Cd(Zn)Te alloyed QDs were prepared in aqueous phase under an argon atmosphere using TGA as a capping ligand by the method described in [9]. To understand the influence of Znprecursor nature on the structure of prepared Cd(Zn)Te alloyed QDs two different types of Znprecursors were used (Zn2+ and Zn-TGA complex). The first series of samples were obtained by the injection of ZnSO4 to freshly prepared CdTe nanoclusters and subsequent heating. The second series of samples were obtained by the same methodology using Zn-TGA precursor. One sample of CdTe QDs was used as a control. Using of ZnSO4 as a Zn-precursor leads to the blue shift of absorption and photoluminescence spectra compared to CdTe QDs. This indicates incorporation of Zn2+ in the CdTe QDs structure by the cation-exchange mechanism and formation of Cd(Zn)Te alloyed QDs (Fig. 3A). The anodic peak at 1.48 V (marked as A) and cathodic peak at −0.86 V (marked as C1) were observed at the CV curves recorded for a CdTe QDs colloidal solution (Fig. 3B black line).They correspond to the transition of an electron between the electrode and the energy levels of the CdTe QDs. The calculated values of bandgap energy agree with that determined using spectroscopic data. At the CV curve recorded for Cd(Zn)Te alloyed QDs additional cathodic peak at -1.13 V was observed (Fig. 3B blue line). It can be attributed to the electrochemical activity of the energy levels formed by Zn2+ on top of CdTe QDs valence band level and the formation of Cd(Zn)Te alloyed QDs. According to the atomic emission spectroscopy analysis, the ratio between cadmium and zinc in the Cd(Zn)Te alloyed QDs is 0.93/0.07. In the case of Zn-TGA precursor formation of CdTe/ZnS QDs with thick ZnS shell occurs. This is evidenced by the red shift of absorption and photoluminescence spectra, a disappearance of the electrochemical signal from CdTe core and appearance of ZnS shell signal at cyclic voltammograms instead. At Fig. 3B (red line) only one cathodic peak at −1.20 V was detected in the studied range of potentials. Place for Figure 3 We can conclude that changes in the synthetic procedure play an important role in the formation of Cd(Zn)Te alloyed QDs. 3.3.
Impact of CdS shell growth on the CdTe QDs band structure parameters.
It is well known that the thermal decomposition of sulfur-containing stabilizing ligands favors the formation of CdS shell on the CdTe core [14]. Therefore, the thickness of the CdS shell can be controlled by variation of heat treatment process duration. It was shown that during the heat treatment for 6 hours occurs formation of a CdS shell that completely covers the CdTe QDs core [5]. To evaluate the possibility of the detection of the electrochemical signal from CdS shell the series of CdTe/CdS core/shell QDs were prepared by the refluxing of TGA-stabilized CdTe QDs colloidal solution at 100°C for 1 (sample 1) and 9 hours (sample 2). Fig. 4A shows normalized absorption and PL spectra of TGA-stabilized CdTe QDs chosen for the electrochemical investigation of the impact of the CdS shell growth on the CdTe band structure. The absorption maximum shifts to longer wavelengths with the increase of heat treatment time from 1 up to 9 hours as a consequence of CdTe core size increasing and CdS shell growth. At CV curve recorded for the sample 1 (black line) an anodic peak at about 1.46 V (A) and less pronounced cathodic peak (C1) at -0.83 V can be observed (Fig. 4B). The less pronounced cathodic peak can be attributed to the partial degradation of QDs after the electron transfer
(oxidation process) [15]. This effect is enhanced by the poor surface passivation as the result of short-time heat treatment.At CV curves recorded for the sample 2 (red line) an anodic peak at about 1.46 V (A), cathodic peak (C1) at -0.89 and addition cathodic peak (C2) at -0.25 V were observed (Fig. 4B). The appearance of the cathodic peak C2 may indicate the electrochemical activity of the CdS shell and corresponds to the conduction band edge energy of CdS. At the same time, the peak C1 becomes more pronounced which confirms the good surface passivation of CdTe QDs by CdS shell. Energy levels of CdTe/CdS QDs that were determined from the CVs are plotted in the inset of Fig. 4B. Place for Figure 4 4.
Conclusions
The influence of the concentration of a stabilizer demonstrates a significant impact on the electrochemical response of the CdTe QDs. Valuable information about changes in the CdTe QDs optical properties and band structure as the result of CdS shell formation, Hg- and Znalloying was obtained using cyclic voltammetry method. Acknowledgments This work was partially supported by the Ministry of Education and (Grants:#0116U001447 "Optically active materials based on metallic nanocrystals embedded into the crystalline and amorphous matrix" “Multifunctional nitrogen-containing heterocyclic antioxidants as effective photodegradation of quantum dots in optically active materials”).
Science of Ukraine and semiconductor and #0116U006958 retarders of process
References [1] V. H. Chu,T. H. L. Nghiem,T. H. Le, D. L.Vu, H. N.Tran, T. K. L. Vu, Synthesis and optical properties of water soluble CdSe/CdS quantum dots for biological applications,Adv. Nat. Sci: Nanosci. Nanotechnol. 3(2) (2012) 025017. [2] N.Van Hung, D. A.Tuan,T. D.Thien, N. D.Phu,D. B. Do, P.Van Vinh, Effect of preparation conditions on optical properties of CdTe quantum dot dispersed in water // VNU J. Sci. : Mathematics-Physics, 28 (1) (2012) 39-45. [3] F. Gao, J. Li, F. Wang, T. Yang, D. Zhao, Synthesis and characterization of high-quality water-soluble CdMnTe quantum dots capped by N-acetyl-L-cysteine through hydrothermal method, J. Lumin., 159 (2015) 32-37. [4] S. Taniguchi, M. Green, T. Lim, The room-temperature synthesis of anisotropic CdHgTe quantum dot alloys: a “molecular welding” effect, J. Am. Chem. Soc., 133(10) (2011) 33283331. [5] N. Doskaliuk, Y. Khalavka, P. Fochuk, Influence of the shell thickness and ratio between core elements on photostability of the CdTe/CdS core/shell quantum dots embedded in a polymer matrix, Nanoscale Res. Lett., 11(1) (2016) 216. [6] J. Wang, Y. Long, Y. Zhang, X. Zhong, L. Zhu, Preparation of highly luminescent CdTe/CdS core/shell quantum dots, Chem. Phys. Chem. 10(4) (2009) 680-685. [7] J. Liu, W. Yang, Y. Li, L. Fan, Y. Li, Electrochemical studies of the effects of the size, ligand and composition on the band structures of CdSe, CdTe and their alloy nanocrystals, Phys. Chem. Chem. Phys., 16(10) (2014) 4778-4788. [8] E. Kucur, J. Riegler, G. A. Urban, T. Nann Determination of quantum confinement in CdSe nanocrystals by cyclic voltammetry, J. Chem. Phys.119(4) (2003) 2333-2337. [9] O. O. Tynkevych, K. Ranoszek-Soliwoda, J. Grobelny, O. V. Selyshchev, Y. B. Khalavka Spectroscopic and electrochemical monitoring of band structure changes during the alloying of CdTe QDs by Hg2+ ions, Mater. Res. Express 3(10) (2016) 1-9. [10] A. L. Rogach, M. T. Harrison, S. V. Kershaw, A. Kornowski, M. G. Burt, A. Eychmüller, H. Weller, Colloidally Prepared CdHgTe and HgTe Quantum Dots with Strong Near‐Infrared Luminescence, Phys. Status Solidi B22(1)(2001) 153-158. [11] C. H Ma, Y. M. Shi, L. B. Chen, Y. F. Wu, X. Chen Electrochemiluminescence sensor using gold-nanoparticle modification combining mercaptoacetic acid-assembly. Anal. Methods, 4(12) (2012). 4096-4100.
[12] G. K. Ziyatdinova, G. K. Budnikov, V. I. Pogorel’tsev Determination of captopril in pharmaceutical forms by stripping voltammetry. Journal of Analytical Chemistry, 61(8) (2006) 798-800. [13] A. B. F. Vitoreti, O. Abrahão, R. A. D. Gomes, G. R. Salazar-Banda, R. T. Oliveira Electroanalytical determination of captopril in pharmaceutical formulations using boron-doped diamond electrodes, Int. J. Electrochem. Sci. 9 (2014) 1044-1054. [14] F. O. Silva, M. S. Carvalho, R. Mendonça,W. A. Macedo, K. Balzuweit, P. Reiss, M. A. Schiavon, Effect of surface ligands on the optical properties of aqueous soluble CdTe quantum dots, Nanoscale Res. Lett. 7(1) (2012) 536-546. [15] X. Ma, A. Mews, T. Kipp, Determination of electronic energy levels in type-II CdTecore/CdSe-shell and CdSe-core/CdTe-shell nanocrystals by cyclic voltammetry and optical spectroscopy,J. Phys. Chem. C, 117(32)(2013) 16698-16708. Figures and captions Fig. 1 A. Normalized absorption and PL spectra of TGA-stabilized CdTe and series of Cd(Hg)Te alloyed QDs colloidal solutions; B. Cyclic voltammograms recorded for TGA-stabilized CdTe, CdHg(2)Te alloyed and CdHg(4)Te alloyed QDs colloidal solutions. The scan rates were 100 mV/s; Inset: Scheme of the band structure of CdTe and Cd(Hg)Te alloyed QDs, calculated from CV data.
Fig. 2 A. Normalized Absorption and PL spectra of CdTe and Cd(Hg)Te alloyed QDs stabilized with excess of TGA; B. Cyclic voltammograms recorder for thioglycolic acid-stabilized CdTe and Cd(Hg)Te alloyed QDs colloids and 20 mg/L TGA. The scan rates were 100 mV/s.
Fig. 3 A Normalized Absorbtion and Photoluminescence spectra of TGA-stabilized CdTe, Cd(Zn)Te alloyed and CdTe/ZnS QDs: B Cyclic voltammograms recorded for TGA-stabilized CdTe, Cd(Zn)Te alloyed and CdTe/ZnS QDs. The scan rates were 100 mV/s; Insert: Plot of valence (VB) and conduction (CB) band edge positions for TGA-stabilized CdTe, Cd(Zn)Te alloyed and CdTe/ZnS QDs obtained from the respective anodic and cathodic peak.
Fig. 4 A. Normalized absorption and PL spectra of TGA-stabilized CdTe and CdTe/CdS QDs; B Cyclic voltammograms recorder for TGA-stabilized CdTe QDs after 1 and 9 hours of heat treatment process. The scan rates were 100 mV/s; Inset: Scheme of the band structure of CdTe/CdS QDs, calculated from CV data.
S0169-4332(18)30998-X https://doi.org/10.1016/j.apsusc.2018.04.044 APSUSC 39043
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
19 September 2017 11 March 2018 5 April 2018
Please cite this article as: O. Tynkevych, Y. Khalavka, Influence of transition metal alloy in gand surface modification of the CdTe quantum dots on their optical properties, band structure and electrochemical activity, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.04.044
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of transition metal alloy in gand surface modification of the CdTe quantum dots on their optical properties, band structure and electrochemical activity OlenaTynkevych, Yuriy Khalavka Department of Solid State Inorganic Chemistry and Nanomaterials, Yuriy Fedkovych Chernivtsi National University, Kotsiubynsky Str. 2, 58012, Chernivtsi, Ukraine Abstract We report a study of CdTe quantum dots modification bythe alloying of with Zn2+ and Hg2+ions and shell growth. Optical and cyclic voltammetry measurements were used to explain changes in the band structure of the particles via shell growth and during the alloying. Influence of stabilizer on the electrochemical behavior of the particles is also discussed. Obtained nanostructures demonstrate photoluminescence in the wider range of wavelengths than pure CdTe QDs. Keywords: CdTe, quantum dots, transition metals, alloying, cyclic voltammetry, band structure. 1.
Introduction
CdTe quantum dots (QDs) due to their unique size- and composition-dependent optical and electrical properties are promising materials for different applications such as chemical sensors, solar elements, LED devices, in vivo and in vitro biosensing imaging etc. In addition to the widely known size- dependent optical properties of CdTe QDs there is an alternative method for tuning QDs optical and electrical properties by varying the QDs composition. Such doped/alloyed CdTe QDs have been produced via composition-dependent transition metals doping/alloying processes. The modifications of the optical and electrical properties of CdTe QDs can be achieved using different techniques. For instance, Chu and Van Hung [1, 2] demonstrated that optical and electrical properties of QDs can be changing by adjusted synthesis conditions. The incorporation of transition metals impurities into QDs is another useful technique used for tuning their optical properties [3, 4] due to the band structure changes. Also as shown by Doskaliuk [5] and Wang [6], prolonged heat treatment of the CdTe colloidal solutions leads to the significant shift of PL maximum to the long-wave region due to the CdS shell growth and the formation of type II core/shell CdTe/ CdS QDs. For a wide range of applications based on CdTe QDs, it is important to understand the effects of synthesis conditions, impurities incorporation and CdS shell growth on the band structure changes. Analysis of Cyclic voltammetry (CV) data of oxidation and reduction reactions involving CdTe QDs enables calculation of the position of CdTe QDs bandgap energy levels and various impurities levels position [7]. In the present work, we summarize application of cyclic voltammetry in the monitoring of band structure changes of CdTe QDs during transition metal alloying and the shell growth. 2.
Material and methods
The absorption and photoluminescence spectra were recorded at room temperature with OceanOptics USB-2000 spectrophotometer. The CV measurements were performed using a computer-controlled Potentiostat/Galvanostat PI-50-1. A three-electrode system with a glassycarbon counter electrode, an Ag/AgCl reference electrode and the platinum electrode as the working electrode was used for the cyclic voltammetry measurements. The cyclic voltammograms were obtained by scanning the potential from – 1.5 to 1.5 V at a scan rate of 100 mV/s. The elemental analysis was carried out on C115M1 Atomic emission spectroscope (AES). 2.1.
Synthesis of QDs
CdTe QDs stabilized by thioglycolic acid (TGA) were synthesized at room temperature in alkali aqueous solution by the reaction of CdSO4 and electrochemically generated H2Te in the presence of thioglycolic acid. The oxygen was removed from the reaction system by bubbling
argon through solutions for 15-20 min. After the reaction, the thermal processing of colloids at 100°C was carried out. Synthesis of Cd(Hg)Te, Cd(Zn)Te alloyed QDs was carried out by addition of different amounts Hg(NO3)2 and ZnSO4 respectively to the freshly prepared CdTe nanocluster colloidal solutions with subsequent heat treatment at 100°C. The chemicals and reagents used in this study: analytical grade 3CdSO4×8H2O of ≥ 99.0 % purity; thioglycolic acid of ≥ 98 % purity (catalog number T3758); NaOH of ≥ 99 purity (S8045); tellurium granular 99.99 % trace metals basis (263303) all from Sigma-Aldrich and analytical grade Hg(NO3)2 and ZnSO4 of ≥ 98% purity. 3.
Results and Discussion
3.1.
CdTe QDs band structure changes during the Hg-alloying.
The TGA-stabilized Cd(Hg)Te QDs were obtained by the addition of Hg2+ ions precursor into the freshly prepared colloidal solution of CdTe nanoclusters and subsequent thermal treatment for 5 hours at 100°C. The amount of added Hg2+ ions was 2% (CdHg(2)Te QDs) and 4% (CdHg(4)Te QDs) of Cd2+ content in the precursor solution. Such alloying leads to the formation of Cd(Hg)Te alloyed QDs by ion-exchange reaction [9, 10]. As a result, the bathochromic shift of PL peak from 588 nm to the near-infrared regions of the spectrum (730 nm) was observed (Fig. 1 A). The appearance of the additional PL peak at 910 nm can be explained by the formation of HgTe shell on top of QDs surface proving complete replacement of surface Cd2+ ions by Hg2+ ions [9]. The absorption maximum is shifted towards longer wavelengths and becomes less pronounced. Such optical properties caused by the changes in the band structure during the transformation of CdTe QDs into Cd(Hg)Te alloyed QDs by ion-exchange reaction [9, 10]. Most likely, the ion-exchange process occurs at the two stages: alloying of CdTe nanoclusters and further alloying of growing QDs during heat treatment. Fig. 1 B shows cyclic voltammograms recorded for a CdTe QDs colloidal solution (black line). The anodic peak at 1.28 V (marked as A1) and cathodic peak at −0.87 V (marked as C) are observed. The calculations of edge positions of the valence band (VB), conduction band (CB) and bandgap values were performed according to the approach described in [8]. Calculated bandgap value of 2.15 eV is in good agreement with the optical band gap obtained from the absorption peak maximum (2.20 eV). At the cyclic voltammograms for the CdHg(2)Te alloyed QDs colloidal solution (Fig. 1 B, blue line) the same oxidation peak at 1.28V, reduction peak at -0.87V and appearance of additional oxidation peak at 0.86 V (marked as A2) is observed. These changes can be explained by the formation of Hg energy levels in the band gap of Cd(Hg)Te alloyed QDs. It is important to note that in the case of CdHg(4)Te QDs (red line), the oxidation peak A1 at 1.28V has completely disappeared. Such effect may identify that many of cadmium surface atoms are replaced by mercury that leads to the formation of the core/shell structure CdTe/Cd(Hg)Te. A similar effect was reported in our previous work [9] for Cd(Hg)Te alloyed QDs. Based on the obtained CV data and calculations we plotted energy levels diagram of CdTe and Cd(Hg)Te alloyed QDs (Fig. 1B. Inset). Place for Figure1 To understand the effect of the stabilizing ligand concentration on the electrochemical activity of TGA-stabilized CdTe and Cd(Hg)Te alloyed QDs have been synthesized according to previously described methods with 100% excess of TGA. The amount of added Hg2+ ions was 5% of Cd2+ content in the solution. As shown in Fig. 2A, the absorption peaks at 550 nm can be seen only for first sample (CdTe QDs). Upon the addition of Hg2+ ions the absorption maximum was disappeared which is typical for Hg-alloyed CdTe QDs [10]. We also observed the bathochromic shift of the PL peaks from 586 nm (for the CdTe QDs) to the infrared region of the spectrum and occurrence of doublepeaks in PL spectra with maxima at 838 and 907 nm (for the Cd(Hg)Te alloyed QDs). This effect can be explained by the formation of HgTe shell on top of CdTe QDs surface as the result of the cation exchange process [9]. Nevertheless, as shown by the CV measurements, the electrochemical signal which corresponds to the electrochemical activity of QDs was not
detected. At the cyclic voltammograms for both samples, only the oxidation peak at 1 V and reduction peak at -0.9 V were observed (Fig. 2B). The observed signal is similar as for the 20 mg/L solution of TGA. The oxidation peak in the range around 1.0-1.3 V attributed to the electrooxidation of the thiol group of TGA [11-13]. This result indicates that excess of TGA leads to the formation of a dense stabilizing ligand layer on top of QDs which hinder detection of the electrochemical response of QDs (Fig. 2B Inset). Place for Figure2 3.2.
CdTe QDs band structure changes during the Zn-alloying
Cd(Zn)Te alloyed QDs were prepared in aqueous phase under an argon atmosphere using TGA as a capping ligand by the method described in [9]. To understand the influence of Znprecursor nature on the structure of prepared Cd(Zn)Te alloyed QDs two different types of Znprecursors were used (Zn2+ and Zn-TGA complex). The first series of samples were obtained by the injection of ZnSO4 to freshly prepared CdTe nanoclusters and subsequent heating. The second series of samples were obtained by the same methodology using Zn-TGA precursor. One sample of CdTe QDs was used as a control. Using of ZnSO4 as a Zn-precursor leads to the blue shift of absorption and photoluminescence spectra compared to CdTe QDs. This indicates incorporation of Zn2+ in the CdTe QDs structure by the cation-exchange mechanism and formation of Cd(Zn)Te alloyed QDs (Fig. 3A). The anodic peak at 1.48 V (marked as A) and cathodic peak at −0.86 V (marked as C1) were observed at the CV curves recorded for a CdTe QDs colloidal solution (Fig. 3B black line).They correspond to the transition of an electron between the electrode and the energy levels of the CdTe QDs. The calculated values of bandgap energy agree with that determined using spectroscopic data. At the CV curve recorded for Cd(Zn)Te alloyed QDs additional cathodic peak at -1.13 V was observed (Fig. 3B blue line). It can be attributed to the electrochemical activity of the energy levels formed by Zn2+ on top of CdTe QDs valence band level and the formation of Cd(Zn)Te alloyed QDs. According to the atomic emission spectroscopy analysis, the ratio between cadmium and zinc in the Cd(Zn)Te alloyed QDs is 0.93/0.07. In the case of Zn-TGA precursor formation of CdTe/ZnS QDs with thick ZnS shell occurs. This is evidenced by the red shift of absorption and photoluminescence spectra, a disappearance of the electrochemical signal from CdTe core and appearance of ZnS shell signal at cyclic voltammograms instead. At Fig. 3B (red line) only one cathodic peak at −1.20 V was detected in the studied range of potentials. Place for Figure 3 We can conclude that changes in the synthetic procedure play an important role in the formation of Cd(Zn)Te alloyed QDs. 3.3.
Impact of CdS shell growth on the CdTe QDs band structure parameters.
It is well known that the thermal decomposition of sulfur-containing stabilizing ligands favors the formation of CdS shell on the CdTe core [14]. Therefore, the thickness of the CdS shell can be controlled by variation of heat treatment process duration. It was shown that during the heat treatment for 6 hours occurs formation of a CdS shell that completely covers the CdTe QDs core [5]. To evaluate the possibility of the detection of the electrochemical signal from CdS shell the series of CdTe/CdS core/shell QDs were prepared by the refluxing of TGA-stabilized CdTe QDs colloidal solution at 100°C for 1 (sample 1) and 9 hours (sample 2). Fig. 4A shows normalized absorption and PL spectra of TGA-stabilized CdTe QDs chosen for the electrochemical investigation of the impact of the CdS shell growth on the CdTe band structure. The absorption maximum shifts to longer wavelengths with the increase of heat treatment time from 1 up to 9 hours as a consequence of CdTe core size increasing and CdS shell growth. At CV curve recorded for the sample 1 (black line) an anodic peak at about 1.46 V (A) and less pronounced cathodic peak (C1) at -0.83 V can be observed (Fig. 4B). The less pronounced cathodic peak can be attributed to the partial degradation of QDs after the electron transfer
(oxidation process) [15]. This effect is enhanced by the poor surface passivation as the result of short-time heat treatment.At CV curves recorded for the sample 2 (red line) an anodic peak at about 1.46 V (A), cathodic peak (C1) at -0.89 and addition cathodic peak (C2) at -0.25 V were observed (Fig. 4B). The appearance of the cathodic peak C2 may indicate the electrochemical activity of the CdS shell and corresponds to the conduction band edge energy of CdS. At the same time, the peak C1 becomes more pronounced which confirms the good surface passivation of CdTe QDs by CdS shell. Energy levels of CdTe/CdS QDs that were determined from the CVs are plotted in the inset of Fig. 4B. Place for Figure 4 4.
Conclusions
The influence of the concentration of a stabilizer demonstrates a significant impact on the electrochemical response of the CdTe QDs. Valuable information about changes in the CdTe QDs optical properties and band structure as the result of CdS shell formation, Hg- and Znalloying was obtained using cyclic voltammetry method. Acknowledgments This work was partially supported by the Ministry of Education and (Grants:#0116U001447 "Optically active materials based on metallic nanocrystals embedded into the crystalline and amorphous matrix" “Multifunctional nitrogen-containing heterocyclic antioxidants as effective photodegradation of quantum dots in optically active materials”).
Science of Ukraine and semiconductor and #0116U006958 retarders of process
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Fig. 2 A. Normalized Absorption and PL spectra of CdTe and Cd(Hg)Te alloyed QDs stabilized with excess of TGA; B. Cyclic voltammograms recorder for thioglycolic acid-stabilized CdTe and Cd(Hg)Te alloyed QDs colloids and 20 mg/L TGA. The scan rates were 100 mV/s.
Fig. 3 A Normalized Absorbtion and Photoluminescence spectra of TGA-stabilized CdTe, Cd(Zn)Te alloyed and CdTe/ZnS QDs: B Cyclic voltammograms recorded for TGA-stabilized CdTe, Cd(Zn)Te alloyed and CdTe/ZnS QDs. The scan rates were 100 mV/s; Insert: Plot of valence (VB) and conduction (CB) band edge positions for TGA-stabilized CdTe, Cd(Zn)Te alloyed and CdTe/ZnS QDs obtained from the respective anodic and cathodic peak.
Fig. 4 A. Normalized absorption and PL spectra of TGA-stabilized CdTe and CdTe/CdS QDs; B Cyclic voltammograms recorder for TGA-stabilized CdTe QDs after 1 and 9 hours of heat treatment process. The scan rates were 100 mV/s; Inset: Scheme of the band structure of CdTe/CdS QDs, calculated from CV data.