Letter Controlling the growth of ultrasmall CdTe quantum dots and the diffusion of cadmium vacancies: Thermal annealing Noelio O. Dantas ⇑ , Guilherme L. Fernandes, Anielle Christine A. Silva ⇑ Laboratório de Novos Materiais Isolantes e Semicondutores (LNMIS), Instituto de Física, Universidade Federal de Uberlândia, Uberlândia, Brazil article info Article history: Received 20 November 2014 Received in revised form 21 February 2015 Accepted 4 March 2015 Available online 12 March 2015 Keywords: Ultrasmall CdTe quantum dots Silicate glass matrix Diffusion of cadmium vacancies Optical properties abstract Ultrasmall CdTe quantum dots (USQDs) were successfully grown in a silicate glass matrix by fusion and after thermal annealing. Growth control of USQDs was investigated by optical absorption (OA), atomic force microscopy (AFM), transmission electron microscopy (TEM) and photoluminescence (PL). A redshift of OA band with increasing thermal annealing time provided evidence of CdTe USQD growth. This increase of average size of the CdTe USQDs was determined by OA spectra, AFM and TEM images. In addi- tion, PL spectra showed that longer thermal annealing times decreased deep levels luminescent intensity from cadmium vacancies (V Cd ) in the CdTe USQDs. This phenomenon occurred because V Cd diffused to the USQDs’ surface with longer thermal annealing times. Therefore, we control the growth of CdTe USQDs as well as the luminescent intensity from surface defects and V Cd as a function of thermal annealing time. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction Cadmium telluride (CdTe) is a II–VI semiconductor with a zinc- blend crystalline structure, exciton Bohr radius of 6.5 nm, absorbs and emits in the infrared optical window with band gap energy of 1.5 eV (826.5 nm) at room temperature [1] has a high melting point of 1041 °C with evaporation starting at 1050 °C. Usually, CdTe shows the p-type semiconductor because the cadmium vacancies (V Cd ) are present [2]. One of the exceptional advantages of semiconductor nanocrys- tals (NCs) is that physical properties are changed by the size and shape. Thus, promising materials simply tuning the absorption and luminescence spectra of the NCs aiming several optical applica- tions [3]. CdTe quantum dots (QDs) are often used as luminescent probes [4–7], solar cells [8,9], photodetectors [10,11] and photovol- taic devices [12,13]. Depending on the application, QDs can be inserted into various systems such as aqueous solutions [14,15], polymer films [16–20], substrates [21–23] and glass matrix [24– 28]. QDs synthesized in glass matrix has several advantages, such as high mechanical and chemical stability, protection of the semi- conductor material against the environment, prevention of agglomeration, and adaptability to device manufacturing processes [29,30]. These characteristics allow QDs to have potential applica- tions in optoelectronics and optical cut-off filters [10,30–32]. The lower the quantum dots more the quantum confinement properties are enhanced. But it is noteworthy that this size is related to the Bohr radius of each material. Ultrasmall CdTe QDs are still little studied and mostly obtained by solutions methods [33–35]. Thus, in this study, probably for the first time, we con- trolled the growth of ultrasmall CdTe QDs (USQDs) in a silicate glass matrix synthesized by fusion as a function of thermal annealing time. The optical and morphological properties of the CdTe USQDs embedded in the silicate glass matrix were investigated by optical absorption (OA), atomic force microscopy (AFM), scanning trans- mission electron microscopy (TEM) and photoluminescence (PL). 2. Experimental 2.1. Materials CdTe USQDs were synthesized in SNAB glass matrix with a nominal com- position of 40SiO 2  30Na 2 CO 3  1Al 2 O 3  29B 2 O 3 (mol%) and 4CdTe bulk (wt%). Preparation consisted of melting the powder mixtures in an alumina crucible at 1250 °C for 15 min. The melt was then quickly cooled to 0 °C. Next, thermal anneal- ing was carried out at 555 °C for 0, 2 and 10 h, since, normally we adopted around 30 °C above the glass transition temperature [T g (SNAB) = 525.15 °C = 798.15 K], which is the minimum energy required to cause molecular mobility and the diffu- sion of precursor ions (Cd 2+ and Te 2 ) for the formation/growth of nanocrystals (CdTe USQDs) [35]. 2.2. Characterization Optical absorption (OA) spectra were measured with a UV-3600 UV–VIS–NIR Shimadzu spectrometer operating from 190 to 3300 nm and with a spectral res- olution of 1 nm. Atomic force microscopy (AFM) images of the USQD samples were http://dx.doi.org/10.1016/j.jallcom.2015.03.026 0925-8388/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding authors. E-mail addresses: noelio@ufu.br (N.O. Dantas), aniellechristineas@gmail.com (A.C.A. Silva). Journal of Alloys and Compounds 637 (2015) 466–470 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom