Journal of the Korean Physical Society, Vol. 61, No. 3, August 2012, pp. 444∼448 Photoluminescence in SrHfO 3 Nanocrystals in Relation to Self-trapped Excitons Dong-Hwan Kim and Yun-Sang Lee ∗ Department of Physics, Soongsil University, Seoul 157-743, Korea (Received 24 April 2012, in final form 25 May 2012) We investigated the blue emission of wide bandgap SrHfO3 nanocrystals by using 325-nm photo- excitation. The nanocrystals were initially synthesized by using the combustion method, then, as-synthesized crystals were annealed at different temperatures ranging from 700 ◦ C to 1450 ◦ C. With annealing at higher temperatures, the grain size, as well as the degree of crystallinity of nanocrystals increased. Interestingly, in contrast to the bulk case, the SrHfO3 nanocrystals showed a strong blue emission even at room temperature, which could be explained in the framework of self-trapped excitons. A non-monotonic evolution of the visible emission band with increasing annealing temperature was observed due to the combination of the grain size effect and oxygen vacancy formation. The blue emission observed in SrHfO3 NCs was stronger than that observed in a similar material SrZrO3, which might originate from the high dielectric constants in the former. PACS numbers: 78.20.-e, 77.80.-e, 78.55.-m Keywords: SrHfO 3 , Nanocrystals, Blue emission, Self-trapped excitons DOI: 10.3938/jkps.61.444 I. INTRODUCTION Size effects in electronic materials have become a sub- ject of increasing interest recently. Decreasing dimen- sion in microelectronic and optoelectronic devices re- quires knowledge of the properties of materials in fairly small sizes. In particular, the effects of grain size are well known to play an important role in changing the physical properties, such as the ferroelectricity, phase transitions, and optical properties, of ABO 3 perovskite-type oxides [1-3]. The 5d 0 band insulator perovskite SrHfO 3 has at- tracted much attention as novel electronic material. Because this material has a high melting point (over 2600 ◦ C) [4], it can be used in high-temperature devices such as electrochemical devices due to their having pro- ton conductivity at fairly high temperatures [5]. This material has been suggested as a valuable candidate for high-K gate dielectric oxides. Moreover, its optical prop- erties, such as photoluminescence (PL), have been in- tensively studied in possible relation to applications for new high-temperature-performance optoelectronic de- vices [6,7]. Very recently, a polycrystalline SrHfO 3 sam- ple was observed to exhibit a sizable emission near 2.5 eV at low temperatures with a 325-nm light excitation [8], which may correspond to the well-known blue emission in SrTiO 3 single crystals with defects [9]. However, this blue emission shows a strong temperature dependence, ∗ E-mail: ylee@ssu.ac.kr; Tel: +82-2-820-0404 and its intensity at room temperature is too small to be detectable. This quenching effect has been widely ob- served in other oxides [10,11], and so far has not been well explained. A great need exists for the the room- temperature blue emission of SrHfO 3 to be enhanced for possible applications. Motivated by this, we synthesized SrHfO 3 nanocrystals (NCs). Usually the spatial confine- ment in nanocrystals makes the electron-hole recombina- tion easier. On another point, because the PL signal in oxides is associated with defect/impurity states, a sur- face state with more defects than a bulk state is favorable for visible emission [12]. In this paper, we report blue PL spectra in SrHfO 3 NC prepared by using the combustion method. There have been several studies on the rare-earth-ion (e.g., Ce 3+ )- doped SrHfO 3 NCs, mainly where the emission proper- ties related to the local states in the rare-earth-ions are concerned [13,14]. To our knowledge, this is the first time to study the blue emission of undoped SrHfO 3 NCs in itself, prepared by annealing at different temperatures. Their PL behaviors are closely related to the grain size, oxygen vacancies, and thermal annealing. The mecha- nism of the blue emission caused by 325-nm light excita- tion can be understood in the framework of self-trapped excitons. II. EXPERIMENT SrHfO 3 NCs have been prepared by using the com- bustion method, which consists of an exothermic oxydo- -444-