Crystalline nanostructures of heavy metal iodides Laura Fornaro a,n , Ivana Aguiar b , María Pérez Barthaburu a , Alvaro Olivera a , Isabel Galain b , Maia Mombrú b a Grupo de Semiconductores Compuestos, Centro Universitario de la Región Este, Universidad de la República, Ruta 9 and Ruta 15, Rocha, Uruguay b Grupo de Semiconductores Compuestos, Cátedra de Radioquímica, Facultad de Química, Universidad de la República, General Flores 2124, Montevideo, Uruguay article info Keywords: A1. Nanostructures B1. Iodides B2. Semiconducting material B3. Ionizing radiation detectors abstract Heavy metal iodides such as mercuric iodide and bismuth tri-iodide are among the materials with the best properties for room temperature semiconductor detection. The use of nanostructures of these compounds – recently synthesized – as precursors for nucleation of oriented layers, opens new perspectives for layer growth and, through them, the possibility of a new generation of detectors with enhanced imaging performance. The mentioned heavy metal iodides were synthesized in this work in 1-octadecene as solvent, using octadecanethiol (ODT) or trioctylphosphine (TOP) as capping agents. This is the first report of morphology and size control of mercuric iodide and bismuth tri-iodide nanostructures using capping agents. We obtained promising results for mercuric iodide with ODT and for bismuth tri-iodide with TOP, which will start a new trend in nucleation of heavy metal iodides. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Heavy metal iodides (HgI 2 , PbI 2 , BiI 3 ) crystalline layers have been studied for several years as photoconductors for direct and digital X-ray imagers [1–3]. These devices have shown better properties when the layers are highly oriented [4,5]. The trend in improving heavy metal iodide layers orientation, and consequently, X-ray imagers performance, indicates that the main challenge of the development of direct X-ray imagers is to grow the most oriented layer onto a charge collection matrix, which is an amorphous- and non-homogeneous-substrate [1]. This growth requires, first of all, a thorough understanding of the first steps of layer growth, that means, the nucleation and coalescence of the crystalline material onto amorphous substrates. First attempts to control the nucleation process of HgI 2 and BiI 3 were approached by the physical vapor deposition method [6,7]. However, nucleation onto amorphous substrates is one of the most difficult processes to control, because the substrate itself does not impose any crystalline orientation to the nuclei. Previous results led us to foresee the use of other methods of controlling the nucleation of heavy metal iodides onto amorphous substrates, e.g. employing nanoscience [8–10]. In light of these antecedents, the present work reports the synthesis of HgI 2 and BiI 3 nanostructures, with the aim of using them as nuclei for growing oriented layers onto amorphous substrates. 2. Methods We synthesized HgI 2 and BiI 3 nanostructures in 1-octadecene (ODE) with and without capping agents (CAs) following a previous reported method [8]. The employed CAs were octadecanethiol (ODT) and trioctylphosphine (TOP), with a molar ratio to metal of 1:1. Different synthesis conditions were used as detailed in Table 1. For the two iodides, synthesis time and temperature were varied when no CA was employed. The HgI 2 syntheses were performed in one step, and for BiI 3 a two step procedure was carried out. After selecting the best conditions (among the aforementioned) for each iodide, we used ODT and TOP as CAs, in a single step procedure for both, injecting the CA with the metal source. The obtained nanostructures were first characterized by pow- der X-ray diffraction (XRD) using a Rigaku ULTIMA IV Diffract- ometer. Transmission electron microscopy (TEM) studies were carried out on a JEOL 1010 and high resolution studies TEM (HR-TEM) on a JEOL 2100. The presence of CA in the samples was studied by Fourier Transform Infrared Spectroscopy (FTIR) in a Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jcrysgro Journal of Crystal Growth http://dx.doi.org/10.1016/j.jcrysgro.2014.02.025 0022-0248 & 2014 Elsevier B.V. All rights reserved. n Tel.: þ598 44799507. E-mail addresses: [email protected](L. Fornaro), [email protected](I. Aguiar), [email protected](M.P. Barthaburu), [email protected](A. Olivera), [email protected](I. Galain), [email protected](M. Mombrú). Please cite this article as: L. Fornaro, et al., Journal of Crystal Growth (2014), http://dx.doi.org/10.1016/j.jcrysgro.2014.02.025i Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Transcript
Crystalline nanostructures of heavy metal iodides
Laura Fornaro a,n, Ivana Aguiar b, María Pérez Barthaburu a, Alvaro Olivera a, Isabel Galain b,Maia Mombrú b
a Grupo de Semiconductores Compuestos, Centro Universitario de la Región Este, Universidad de la República, Ruta 9 and Ruta 15, Rocha, Uruguayb Grupo de Semiconductores Compuestos, Cátedra de Radioquímica, Facultad de Química, Universidad de la República, General Flores 2124, Montevideo,Uruguay
Heavy metal iodides such as mercuric iodide and bismuth tri-iodide are among the materials with thebest properties for room temperature semiconductor detection. The use of nanostructures of thesecompounds – recently synthesized – as precursors for nucleation of oriented layers, opens newperspectives for layer growth and, through them, the possibility of a new generation of detectors withenhanced imaging performance. The mentioned heavy metal iodides were synthesized in this work in1-octadecene as solvent, using octadecanethiol (ODT) or trioctylphosphine (TOP) as capping agents. Thisis the first report of morphology and size control of mercuric iodide and bismuth tri-iodidenanostructures using capping agents.
We obtained promising results for mercuric iodide with ODT and for bismuth tri-iodide with TOP,which will start a new trend in nucleation of heavy metal iodides.
& 2014 Elsevier B.V. All rights reserved.
1. Introduction
Heavy metal iodides (HgI2, PbI2, BiI3) crystalline layers havebeen studied for several years as photoconductors for direct anddigital X-ray imagers [1–3]. These devices have shown betterproperties when the layers are highly oriented [4,5].
The trend in improving heavy metal iodide layers orientation,and consequently, X-ray imagers performance, indicates that themain challenge of the development of direct X-ray imagers is togrow the most oriented layer onto a charge collection matrix,which is an amorphous- and non-homogeneous-substrate [1]. Thisgrowth requires, first of all, a thorough understanding of the firststeps of layer growth, that means, the nucleation and coalescenceof the crystalline material onto amorphous substrates.
First attempts to control the nucleation process of HgI2 and BiI3were approached by the physical vapor deposition method [6,7].However, nucleation onto amorphous substrates is one of the mostdifficult processes to control, because the substrate itself does notimpose any crystalline orientation to the nuclei. Previous resultsled us to foresee the use of other methods of controlling the
nucleation of heavy metal iodides onto amorphous substrates, e.g.employing nanoscience [8–10].
In light of these antecedents, the present work reports thesynthesis of HgI2 and BiI3 nanostructures, with the aim of using themas nuclei for growing oriented layers onto amorphous substrates.
2. Methods
We synthesized HgI2 and BiI3 nanostructures in 1-octadecene(ODE) with and without capping agents (CAs) following a previousreported method [8]. The employed CAs were octadecanethiol(ODT) and trioctylphosphine (TOP), with a molar ratio to metal of1:1. Different synthesis conditions were used as detailed in Table 1.For the two iodides, synthesis time and temperature were variedwhen no CA was employed. The HgI2 syntheses were performed inone step, and for BiI3 a two step procedure was carried out. Afterselecting the best conditions (among the aforementioned) for eachiodide, we used ODT and TOP as CAs, in a single step procedure forboth, injecting the CA with the metal source.
The obtained nanostructures were first characterized by pow-der X-ray diffraction (XRD) using a Rigaku ULTIMA IV Diffract-ometer. Transmission electron microscopy (TEM) studies werecarried out on a JEOL 1010 and high resolution studies TEM(HR-TEM) on a JEOL 2100. The presence of CA in the sampleswas studied by Fourier Transform Infrared Spectroscopy (FTIR) in a
Shimadzu IRPrestige-21 using a DiffusIR Pike Technology accessoryfor diffuse reflectance.
3. Results and discussion
HgI2 nanostructures composition was confirmed by X-raydiffraction for all the synthesis, with and without CA. Fig. 1 showsa representative X-ray diagram, in comparison to the α-HgI2 ICSDfile 68262.
Under H1 conditions we obtained nanostructures of irregularmorphology. The smallest and the most uniform structures wereobtained when the synthesis was performed during 240 min (H2sample). Under these conditions, we obtained long structures self-assembled into squares (30–100 nm in width), each containingrounded nanostructures (2–30 nm in diameter), (see Fig. 2a and b).
After determining the best synthesis conditions without CA, wesynthesized HgI2 nanostructures with ODT and with TOP. Synth-esis performed using ODT yielded smaller nanostructures withtwo different morphologies; nanotubes (5–15 nm in diameter),and rounded nanostructures (4 nm in size) that grew in the [3 0 5]direction. Both structures can be seen in Fig. 2c and d. However,when we used TOP we obtained nanostructures of irregularmorphology; consequently, no further work was performed withthis CA for HgI2.
FTIR spectra allowed us to confirm that ODT is present in theH3 sample, which can be seen in Fig. 3. The peaks between 800and 600 cm�1 correspond to the C–S bond, while the observedpeak at 1463 cm�1 is assigned to ODT, but no vibrational assign-ment was obtained from the reference spectrum [11].
For BiI3 samples, compound composition was also confirmed bypowder X-ray diffraction. A representative X-ray diffractogram ofsample B1 in comparison to the ICSD file 53634 can be observed inFig. 4.
Temperature influence on characteristics of BiI3 nanostructureswas studied, with B1 conditions yielding the best results. Under B1conditions, rounded nanostructures and rods were obtained. Thediameters of the rounded particles are in the range of 5–15 nm,and the widths of the rods are in the range of 10–20 nm. The BiI3nanostructures obtained are shown in Fig. 5a and b. HR-TEM wasperformed on these structures for studying their crystallinity,revealing that rounded nanostructures grew in the [2 0 2] direc-tion. When ODT was added to the reaction, we did not obtain pureBiI3. The presence of structures without iodine was determined byenergy dispersive spectroscopy (EDS), possibly due to the incom-plete synthesis of BiI3; this result has already been observed inprevious experiments [9]. In order to obtain BiI3 using this CA, wealso increased the synthesis time. In spite of this, the desiredcompound still failed to synthesize. However, when using TOP asCA, BiI3 rounded nanostructures (5–10 nm in diameter) wereobtained, which can be seen in Fig. 5c.
We deduced the presence of TOP in the B4 sample fromcomparison of the B1 with the B4 spectrum, and taking into accountFig. 4. X-ray diffractogram of B1 sample and the BiI3 ICSD file.
Fig. 5. a and b – TEM images of B1 sample, c – HR-TEM image of B1 sample, d – TEM image of B4 sample and e – HR-TEM image of B4 sample.
L. Fornaro et al. / Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎ 3
Please cite this article as: L. Fornaro, et al., Journal of Crystal Growth (2014), http://dx.doi.org/10.1016/j.jcrysgro.2014.02.025i
the TOP reference spectrum [12]. B1 and B4 spectra are shown inFig. 6.
It should be mentioned that the heavy metal iodides, objectiveof the present work, have high vapor pressure below their meltingpoint; therefore sublimation is a considerable obstacle that shouldbe overcome when observing them in TEM. This is the reason fornot having HR-TEM images of all the samples.
We can explain the different morphologies obtained for HgI2nanostructures synthesized with CA by the “Hard and soft acidbase theory” (HSAB theory). Hg2þand I� are soft acid and baserespectively. The best choice of a CA for HgI2 nanostructuresshould be a soft base, like the RS� (from the ODT) and the R3P(from the TOP) are. The preference of HgI2 for ODT against TOP isbased on the anti-symbiosis principle which establishes that a softbase (I�) attached to a soft center (in this case Hg2þ) can lowerthe affinity for another ligand, a new soft base. This only occurswhen the previous ligand is attached in a coordination site transFig. 6. IR spectrum of B2 and B4 samples.
Fig. 7. Scheme of nanostructures interacting with CAs, a – HgI2 rounded nanostructures interacting with ODT, b – HgI2 nanotubes interacting with ODT and c – BiI3nanostructures interacting with TOP.
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[13]. TOP is a larger CA (it has three aliphatic chains), whichincreases the difficulty of placing all the molecules in cis positions,decreasing its affinity for the Hg2þ . The anti-symbiosis effect isreflected in the bigger HgI2 structures obtained with TOP.
However, when ODT (a single aliphatic chain CA) was employed inthe HgI2 nanostructures synthesis, two morphologies were obtained:rounded nanostructures (smaller than those obtained without CA) andnanotubes. It can be concluded that when ODT is employed as CA, itcontrols the size of HgI2 nanostructures. However, it is not an efficientCA for controlling the morphology. Nevertheless, this statementshould be proven by studying different HgI2:CA molar ratios.
The HASB anti-symbiosis principle cannot be employed to describethe results obtained for BiI3 nanostructures. In this particular case,because Bi3þ is a borderline acid, the HSAB principle cannot strictlypredict the chemical route for this synthesis. Based on other experi-mental observations we suggest the following route for the obtainedBiI3 nanostructures. The BiI3 formation in ODE is a slow processcompared to the HgI2 formation. This fact allows the CA molecules toparticipate as intermediate in the first steps of the mechanism (in Bi3þ
and I� aggregation). In the ODT case, the CA molecules bond to someBi3þ centers before completing the reaction, yielding a mixture of BiI3and an intermediate compound, which has not yet been identified.When TOP was employed, the intermediate compound was notobtained in the final product, and we only identified BiI3. This factcan be explained by the steric impediment that TOP molecule haswhen bonding to the Bi3þ center. BiI3 rounded and smaller nanos-tructures were obtained due to TOP bonding to the growing particle,which deters the nanostructure growth, controlling the process veryefficiently.
In Fig. 7a scheme of how CAs control morphology and size ofthe growing nanostrucutres is shown.
Controlling nanostructures size and morphology is an impor-tant topic for the development of new technological applications.For nucleation, nanostructures should have uniform size distribu-tion, determining high homogeneity when depositing them ontosubstrates. Furthermore, uniform morphology must be addressedin order to control the coalescence of nanostructures.
BiI3 nanotubes and PbI2 nanotubes or closed-cage particles synth-esis using WS2 nanotubes as templates, or using electron beams fromTEM for such purposes were previosuly reported [14]. But, to ourknowledge there are no other reports of heavy metal iodide nanos-tructures synthesis with CA. The structures reported in this article aretemplate-free, present a smaller size, and have better uniformity thanthe BiI3 and HgI2 structures synthesized by similar methods [8–10,15].Our results represent a significant improvement in the development ofheavy metal iodides nanostructures synthesis, and are a first step inthe thorough study of nucleation of these compounds.
Further studies will be devoted to decrease size dispersion, andto deposit the nanostructures onto amorphous substrates, in orderto obtain an oriented nanostructured layer.
4. Conclusions
We obtained crystalline nanostructures for both iodides. Whenwe used ODT for HgI2 and TOP for BiI3 as CAs, we achieved morecontrol of the nanostructures' size and morphology.
This is the first report of heavy metal iodides synthesis usingCAs and represents a step towards controlling nanostructures'properties, in order to use them as nuclei for oriented or epitaxialgrowth.
Future work will be conducted to reach higher uniformity innanostructures' size and crystalline orientation, seeking for moreappropriate nuclei for the study of nucleation and coalescence ofthe materials.
Acknowledgments
LNNano for the electron microscopy work that has beenperformed with the JEM 2100 ARP microscope of the LME/LNNano,CNPEM, Campinas, Brazil; LABNANO, Centro Brasileiro de Pesqui-sas Fisicas, CBPF, Rio de Janeiro, Brasil, LabMic, UniversidadeFederal de Goiás, GO, Brazil; Prof. Dr. Jesiel F. Carvalho, Grupo deCristalografía e Materiais, Instituto de Física, Universidade Federalde Goiás, GO, Brazil, Crysmmat-Lab/DETEMA, Facultad de Química,Universidad de la República, Uruguay. Agencia Nacional de Inves-tigación e Innovación (ANII), Comisión Sectorial de InvestigaciónCientífica (CSIC) and Programa de Desarrollo de las CienciasBásicas (PEDECIBA) for funding.
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