Structural Influence of Erbium Centers on Silicon Nanocrystal Phase Transitions Robert A. Senter, 1 Cristian Pantea, 2 Yuejian Wang, 2 Haozhe Liu, 3 T. Waldek Zerda, 2 and Jeffery L. Coffer 1 1 Department of Chemistry, Texas Christian University, Fort Worth, Texas 76129, USA 2 Department of Physics, Texas Christian University, Fort Worth, Texas 76129, USA 3 HPCAT, Argonne National Laboratory, Argonne, Illinois 60439, USA (Received 12 August 2003; revised manuscript received 28 June 2004; published 18 October 2004) Two different types of erbium-doped silicon nanocrystals, along with undoped, oxide-capped Si dots, are employed to probe the impact of the impurity center location on phase transition pressure. Using a combination of high pressure optical absorption, micro-Raman, and x-ray diffraction mea- surements in a diamond anvil cell, it is demonstrated that the magnitude of this phase transition elevation is strongly dictated by the average spatial location of impurity centers introduced into the nanocrystal along with the interfacial quality of the surrounding oxide. DOI: 10.1103/PhysRevLett.93.175502 PACS numbers: 62.50.+p, 61.72.Tt Often scrutinized for their useful size-dependent opti- cal properties, semiconductor nanocrystals also exhibit intriguing structural phenomena, including melting point depression [1] and phase transition pressure elevation behavior [2,3]. Such characteristics arise from the neces- sity of finite systems to minimize surface energetics dur- ing the transformation event [4]. Silicon (Si) nanocrystals are a particularly important case of such materials, as it has been established that a shape change accompanies the first order phase transition, along with disruption of the Si  SiO 2 interface [3]. In this Letter, we demonstrate that the magnitude of this phase transition elevation is dictated by the average spatial location of impurity cen- ters introduced into the nanocrystal along with the inter- facial quality of the surrounding oxide. One particular type of Si nanophase material of current interest involves a crystalline Si host containing erbium impurity centers. Erbium is of interest because of the  4 I 13=2 ! 4 I 15=2  ligand field transition and the resulting luminescence band at 1:54 m which lies at an absorp- tion minimum for silica based optical fibers and glasses [5,6]. A Si-based nanoscale light emitter whose lumines- cence originates from Si exciton-mediated energy trans- fer with rare earth centers such as Er 3 could prove useful for the construction of a monolithic Si-based optoelec- tronic device. In our laboratory, two different types of doped Si nanocrystals have been synthesized: one involv- ing a random distribution of erbium centers throughout the nanocrystal [7]; the other forces erbium into a loca- tion preferentially enriched near the surface [8]. The fact that we can produce Si nanocrystals containing erbium in two distinctively different structural environments pro- vides a useful comparison as to the role of a rare earth impurity center on the phase behavior of this technologi- cally crucial material. Bulk crystalline Si has a diamond cubic crystal struc- ture at atmospheric pressure and ambient temperatures, and transforms from cubic to the -tin structure at ap- proximately 12 GPa [9–12]. At 13 GPa this phase is converted to a body-centered orthorhombic structure termed Imma [13,14], followed by a transformation at 16 GPa whereby the Si adopts a primitive hexagonal structure [15,16]. Convenient experimental probes of the first order phase transition exist, including optical absorp- tion in the visible region of the spectrum. As the semi- conducting cubic phase of crystalline Si is transformed to the metallic -Sn (or primitive hexagonal in the case of nanocrystals), a loss of transparency occurs with a cor- responding increase in the optical density of the sample [17]. Raman spectroscopy serves as a complementary probe to this type of measurement, as the diminution in intensity of the 520 cm 1 phonon for the cubic phase of Si can be monitored [18]. Hence these experiments also address an unanswered question as to the impact of the Er 3 centers on the structural integrity of the Si nanocrystal. Nanocrystals of both randomly dispersed erbium- doped Si and Si nanocrystals with erbium-rich surfaces were prepared by the controlled pyrolysis of diluted Si 2 H 6 in He along with vapor of the compound Erthmd 3 at 1000  C, achieving erbium incorporation levels of ap- proximately 2 at.% [7,8]. As a control, undoped Si nano- crystals were prepared by employing identical reaction conditions during synthesis, except for the deliberate absence of the erbium source compound. These Si- containing nanocrystals were structurally characterized by a combination of transmission electron microscopy [7,8,19], selected area electron diffraction, energy disper- sive x-ray analysis [7,8,19], and extended x-ray absorp- tion fine structure methods (EXAFS) [8,20]. The average diameter of the undoped Si nanocrystals was 18 nm, the randomly dispersed Er-doped Si NCs was 6 nm, and that of the erbium surface-enriched product was 26 nm; particle size dispersity is on the order of 25%. Pressure is applied to these samples with the use of a diamond anvil cell of Merrill-Bassett design (for Raman) or com- pact cylinder (for optical absorption), with ethylene gly- col or methanol/ethanol/water used as a pressure- VOLUME 93, NUMBER 17 PHYSICAL REVIEW LETTERS week ending 22 OCTOBER 2004 175502-1 0031-9007= 04=93(17)=175502(4)$22.50  2004 The American Physical Society 175502-1