Jin-Han
Lin
a,
Yong-Jie
Huang
a,
Yan-Ping
Su
a,
Che-An
Liu
a,
Rupesh S.
Devan
a,
Ching-Hwa
Ho
b,
Yi-Ping
Wang
b,
Hung-Wen
Lee
a,
Chun-Ming
Chang
a,
Yung
Liou
c and
Yuan-Ron
Ma
*a
aDepartment of Physics, National Dong Hwa University, Hualien 97401, Taiwan, R.O.C. E-mail: ronma@mail.ndhu.edu.tw; Fax: +886-3-8633690; Tel: +886-3-8633706
bGraduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei 106, Taiwan, R.O.C.
cInstitute of Physics, Academia Sinica, Taipei 11529, Taiwan, R.O.C.
First published on 18th January 2012
We explore the structural, electronic, photoluminescent, metallic, and semiconducting characteristics of 2D pure Zn-metal nanoplates. The 2D pure Zn-metal nanoplates are synthesized by the hot-filament metal-oxide vapor deposition (HFMOVD) technique. They have an average diameter and thickness of ∼520 and ∼144 nm, respectively. The results of the electronic and crystalline structure studies reveal the 2D nanoplates to be pure Zn hexagonal crystals, which can provide a wide-range photoluminescence from ultraviolet (UV) to red light emissions at room temperature. The measured valence-band and the calculated band-structure of the 2D pure Zn-metal nanoplates verify that the UV and blue light arise from the 3d–sp interband transitions, while the green, yellow, and red lights come from the valence-conduction interband transitions at a bandgap that is only present in the 2D nanoplates. Therefore, the 2D pure Zn-metal nanoplates possess not only metallic, but also semiconducting characteristics.
The structural morphology and electronic and crystalline structures of the 2D pure Zn-metal nanoplates were examined by field-emission scanning electron microscopy (FESEM), X-ray photoemission spectroscopy (XPS) and by X-ray diffractometry (XRD). PL spectroscopy showed a wide-range of light emissions at room temperature, from ultraviolet (UV) to red, for the 2D pure Zn-metal nanoplates. In comparison with the energy separation ( EF − E3d ≈ 10.2 eV) between the EF and 3d core-level (E3d) of bulk Zn metals,26XPS spectroscopy displayed the EF of the 2D pure Zn-metal nanoplates to be lowered; namely, the EF–E3d energy separation (∼5.5 eV) of the 2D pure Zn-metal nanoplates is relatively small. Therefore, according to Mooradian's model,1 the small EF–E3d energy separation allows the occurrence of UV and blue light PL emissions. The band structures of the 2D pure Zn-metal nanoplates are determined by performing first-principle calculations within the framework of density functional theory (DFT).27,28 The calculated band structures verify that the 2D pure Zn-metal nanoplates possess not only metallic but also semiconducting characteristics. The sp band is taken apart to form a bandgap, which could also provide green and red light emissions. Therefore, in terms of solar cells, the metallic and semiconducting properties of the 2D pure Zn-metal nanoplates offer a huge opportunity for photovoltaic manufacturing.
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Fig. 1 (a) FESEM images showing the top views of a large area array of 2D pure Zn-metal nanoplates on a Si substrate. (b) High-magnification FESEM image showing the side view (70° to the surface normal direction) of 2D pure Zn-metal nanoplates. |
Quantitative analyses of the electronic structures and chemical properties of the 2D pure Zn-metal nanoplates were performed using XPS. Fig. 2(a) illustrates the Zn 2p XPS spectra for the 2D pure Zn-metal nanoplates. The Zn 2p XPS spectra were decomposed using Voigt function fitting to precisely determine the features of the two peaks for Zn 2p3/2 and Zn 2p1/2. They are located at binding energies of 1022.0 and 1045.1 eV, respectively, and the energy separation between them is 23.1 eV. Note that the Zn 2p3/2 peaks at 1022.0 and 1023.0 eV represent pure Zn0 atoms and Zn2+ cations, respectively.29 The XPS results indicate that these 2D hexagonal nanoplates are composed of pure Zn metals only, including no zinc oxides (ZnO) at all. The XRD patterns are essentially the fingerprints of crystalline materials and can reveal details of the structure and the crystal growth formation during synthesis. Fig. 2(b) shows the XRD spectra and Rietveld refinement results for the 2D pure Zn-metal nanoplates. The diffraction peaks at 2θ = 36.3°, 39.1°, 43.2°, and 54.4° correspond respectively to the (002), (100), (101) and (102) lattice planes of the hexagonal Zn crystal in the space groupP63/mmc (JCPDS card no. 65-3358). The refined parameters are obtained for lattice constants of a = b = 2.66638 Å and c = 4.94881 Å, α = β = 90°, γ = 120°, χ2 = 1.901, Rp = 13.31% and Rwp = 20.64%, respectively.
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Fig. 2 (a) XPS spectra of the Zn 2p3/2 and Zn 2p1/2 core levels of the 2D pure Zn-metal nanoplates. The XPS spectra are decomposed via Voigt function fitting. (b) XRD patterns and Rietveld refinement of the 2D pure Zn-metal nanoplates. |
Fig. 3(a) depicts the room-temperature PL spectra of the 2D pure Zn-metal nanoplates. The UV- and visible-light emissions can be clearly observed for a wide range of wavelengths from 312 to 700 nm. There are two main asymmetrical spectral bands centered at 400 and 458 nm, which correspond to the energy states of 3.1 ( E2) and 2.7 (
E3) eV. The shoulder band located at 362 nm corresponds to the energy state of 3.43 eV (
E1). Note that the wavelengths of 312, 500, 600 and 700 nm are reflected by the energies of 3.97, 2.48, 2.07 and 1.77 eV, respectively. As is commonly known, PL spectroscopy is a good optical method for probing the impurities, defects, and bandgaps in semiconductor materials. However, since the 2D pure Zn-metal nanoplates are comprised of only pure metallic Zn atoms, the PL emissions do not reflect the semiconductor properties alone. Fig. 3(b) shows a schematic representation based on Mooradian's model.1 The interband transitions are among the 3d (the region highlighted in yellow), sp-conduction bands and EF for metals at room-temperature. Electrons lying in the 3d band can be excited by an incident laser to move to the unoccupied levels of the sp-conduction band above the EF. The excited electrons then undergo nonradiative transitions to the EF. The electrons at the EF must drop back to the 3d band, so that room-temperature PL emissions of metals occur, releasing energy during the radiative transitions. In this study, the emitted UV and blue light emissions come from the factors so that the electrons at the EF drop to the upper levels (such as the E1 and E2 states) in the 3d band. However, the energy separation between the EF and the 3d band of the 2D pure Zn-metal nanoplates may be too large to generate the green, yellow and red light emissions. Mooradian's model cannot explain the occurrence of all PL emissions. We need to examine the valence-band and band-structure of the 2D pure Zn-metal nanoplates to further understand the occurrence of the green, yellow and red light emissions.
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Fig. 3 (a) Room-temperature PL spectra of the 2D pure Zn-metal nanoplates, where E1, E2, and E3 represent the energy states of 3.43, 3.1, and 2.7 eV, corresponding to 362, 400, and 458 nm, respectively. (b) Schematic representation of the excitation and recombination transition among the 3d (region highlighted in yellow), sp-conduction bands and EF. (c) The XPS spectra of the valence band, EF, 4s and 3d bands of the 2D pure Zn-metal nanoplates. |
Zn has an electronic configuration of [Ar]3d104s2. The outer 4s electrons easily become free electrons and are dispersed in a wide range of energies. The 3d electrons are tightly bound and relatively dispersed in a narrow range of energies. The XPS spectra in Fig. 3(c) show the valence band, EF, 4s and 3d bands of the 2D pure Zn-metal nanoplates. The 4s band has a small but broad peak, while the 3d band has a large peak. The EF is determined at the binding energy of 5.4 eV, and the 3d band peak is centered at 10.9 eV. Therefore, the EF–E3d energy separation between the EF and the center of the 3d band peak is 5.5 eV, which is much larger than the energy (= 4.66 eV) of the incident 266-nm laser. However, from the inset to Fig. 3(c), which displays the detailed electronic structure from the EF to the upper levels of the 3d band, it is evident that the E1 and E2 states lie at the tail of the 3d band. Actually, the E1 and E2 states are the upper levels of the 3d band, and the EF–E1 and EF–E2 energy separations are 3.4 and 3.2 eV, respectively. Hence, only the electrons in the upper levels of the 3d band can be excited to move to the sp conduction band, and the E1 and E2 states are able to provide the UV and blue light emissions. However, the E3 state does not lie in the tail. In other words, the E3 state is not on one of the upper levels, so it cannot provide any PL emissions. Therefore, we need to determine where the green, yellow and red light emissions come from.
Fig. 4 shows the calculated band structure of the 2D pure Zn-metal nanoplates. As can be seen in the inset to Fig. 4, the Brillouin zone is a 2D hexagonal reciprocal lattice. The 3d electrons lie in narrow bands (indicated by the green curves) around 5.5 eV with a high density of states, while the sp conduction bands (indicated by the blue curves) are relatively dispersed with a lower density of states. In the range from the K-point to the Γ-point, the energy separations between the EF and the upper levels of the 3d band are slightly smaller than the energy of the incident laser. Electrons in the upper levels of the 3d band are excited to the sp band, so some 3d–sp interband transitions (indicated by the red arrows) occur. The 3d–sp interband transitions can cause UV and blue light emissions, confirming the PL results shown in Fig. 3(a). However, in two ranges, from the Γ-point to the M-point and from the M-point to the K-point, the sp band is taken apart to form a bandgap, indicating that the 2D pure Zn-metal nanoplates also possess semiconducting characteristics. The bandgaps are ≤2.7 eV (= E3), so electrons in the valence band below the EF are easily excited from the valence band to the conduction band, the so-called valence–conduction interband transitions (indicated by the orange arrows). The excited electrons transit nonradiatively to the M-point, because the lowest valence level above the EF is located at the M-point. When the M-point electrons return to the valence band, they can emit the green, yellow and red light emissions (as shown in Fig. 3(a)), causing the release of energy during the radiative transitions. In addition, we found that the 2D pure Zn-metal nanoplates have both p- and n-type semiconducting properties in the range from the M-point to the Γ-point, and from the M-point to the K-point, respectively, suggesting that there is a p–n junction between these two ranges. Consequently, the 2D pure Zn-metal nanoplates not only demonstrate metallic, but also p- and n-type semiconducting characteristics, which give the 2D pure Zn-metal nanoplates excellent potential for use in optoelectronic nanodevices such as light-emitting diodes (LEDs) and solar cells.
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Fig. 4 Calculated band structure of the 2D pure Zn-metal nanoplates. The red and orange arrows indicate the 3d–sp and valence–conduction interband transitions, respectively. The Brillouin zone of a 2D hexagonal reciprocal lattice is shown in the inset. |
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