Photoabsorption Tolerance of Intrinsic Point Defects and Oxidation in

Dec 14, 2016 - Black phosphorus quantum dots (BPQDs) exhibit excellent optical and photothermal properties and promising applications in optoelectroni...
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Photoabsorption Tolerance of Intrinsic Point Defects and Oxidation in Black Phosphorus Quantum Dots Xianghong Niu, Huabing Shu, Yunhai Li, and Jinlan Wang* Department of Physics, Southeast University, Nanjing, 211189, China S Supporting Information *

ABSTRACT: Black phosphorus quantum dots (BPQDs) exhibit excellent optical and photothermal properties and promising applications in optoelectronics and biomedicine. However, various intrinsic structural defects and oxidation are nearly unavoidable in preparation of BPQDs and how they affect the electronic and optical properties remains unclear. Here, by employing time-dependent density functional theory, we reveal that there are two types of photoabsorption in BPQDs for both point defects and oxidation. A close structure-absorption relation is unraveled: BPQDs are defect-tolerant and show excellent photoabsorption as long as the coordination number (CN) of defective P atoms is 3. By contrast, the unsaturated or oversaturated P atoms with CN ≠ 3 create in-gap-states (IGSs) and completely quench the optical absorption. An effective way to eliminate the IGSs and repair the photoabsorption of defective BPQDs via sufficient hydrogen passivation is further proposed.

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defective BPQDs with CN = 3 are defect-tolerant and show excellent photoabsorption, while the unsaturated or oversaturated P atoms with CN ≠ 3 create IGSs and greatly damage the optical absorption. More interestingly, unlike MoS228 or BN29 edges suffering from the IGSs, hydrogen passivation can eliminate the IGSs of defective BPQDs and recover the photoabsorption effectively. All calculations were carried out by using the Becke threeparameter Lee−Yang−Parr hybrid (B3LYP)30 exchangecorrelation functional along with the Slater-type double-zata plus polarization (DZP) basis set31 as implemented in the Amsterdam Density Functional program package (ADF2013).32,33 The ground state geometries of BPQDs were first optimized at the DFT level and vibrational frequencies were calculated to verify their dynamical stability. The excitation energies were then calculated using TD-DFT. Ten lowest singlet excited states of BPQDs were taken into consideration. All optimizations were done without any symmetry constraint. Intrinsic Point Defects in BPQDs. Since the as-synthesized BPQDs are rectangular-like20 and BP monolayer is in orthorhombic lattice, we constructed square-like pristine BPQDs with zigzag or armchair edges and with edge length about 2 nm as shown in Figure 1a. The dangling bonds are passivated by hydrogen atoms. Seven different configurations were considered: divacancy, zigzag or armchair reconstructed edge, interstitial with or without H passivation, and zigzag edge or armchair edge with partial H passivation (see Figure 1a (ii−

ingle and few-layer black phosphorus (BP) has attracted worldwide research interest1−4 for its combination advantages of tunable direct bandgap,5,6 moderate on/off ratio,7,8 and sufficiently high carrier mobility,9,10 which are vital for semiconductor devices. Particularly, few-layer BP exhibits high in-plane anisotropy in optical properties,11−13 electrical conductance,14 mechanical properties,15−18 owing to the distinct puckered structure. Besides, BP quantum dots (BPQDs) have also been successfully synthesized through chemical methods19−22 and exhibited many fascinating properties such as UV/vis absorption,20 fluorescence,19,20 nearinfrared photothermal conversion performance, and biocompatibility.21 Size-dependent optical absorption and emission spectra and excellent photoabsorption in larger pristine BPQDs have also been predicted theoretically.23 These intriguing features make BPQDs promising candidates in electronics, optoelectronics, biomedicine, and so on. On the other hand, the presence of defects possibly gives rise to in-gap-states (IGSs), which normally serve as trap states and greatly degrade the performance of devices.24 Therefore, to a large extent, the success of materials in many fields is partially attributed to their tolerance to intrinsic structural defects and environment.24 It has been widely reported that intrinsic point defects and oxidation are inevitable in preparation of BP nanosheets25−27 and BPQDs19−22 using wet chemistry synthesis. However, how they influence the electronic and optical properties of BPQDs remains unclear. In this work, we provide a detailed theoretical study on this issue by using timedependent density functional theory (TD-DFT). Very interestingly, no matter for point defects (PDs) or for oxidization defects (ODs), there always exist two kinds of photoabsorption in defective BPQDs depending on the coordination number (CN) of P atoms. More specifically, the © XXXX American Chemical Society

Received: October 25, 2016 Accepted: December 14, 2016 Published: December 14, 2016 161

DOI: 10.1021/acs.jpclett.6b02486 J. Phys. Chem. Lett. 2017, 8, 161−166

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Figure 1. (a) Top and side views of the optimized ground state structures of pristine and defective BPQDs: (i) pristine, (ii) divacancy, (iii) zigzag reconstructed edge, (iv) armchair reconstructed edge, (v) interstitial, (vi) interstitial with H passivation, (vii) zigzag edge with partial H passivation, and (viii) armchair edge with partial H passivation. P and H atoms are colored in orange and white, respectively. The defective P atoms with CN = 3 (CN ≠ 3) are highlighted by magnified green (red) balls. (b) The electronic gaps of pristine and defective BPQDs. (c) Absorption spectrum of pristine and defective BPQDs. A 0.25 eV Lorentzian broadening is employed. For defective BPQDs with the CN ≠ 3, their absorption intensity is totally quenched and thus not shown.

Figure 2. (a) Density of States (DOS) of pristine and intrinsic PD BPQDs. Black lines represent total DOS, red refers to DOS of defective P atoms. The Fermi level is denoted by the purple line. All states are broadened with a Lorentzian width of 0.01 eV. (b) Illustration of the passivation process of unsaturated bond: switching CN from 2 to 3, and the dissociation process of oversaturated bond: switching CN from 4, 5, to 3 after ventilation with hydrogen. The defective P atoms with CN = 3 (CN ≠ 3) are highlighted by magnified green (red) balls. (c) Representative isosurfaces of wave functions of HOMO and LUMO in BPQDs: (i) pristine, (iii) zigzag reconstructed edge, and (iii-H) zigzag reconstructed edge with hydrogen passivation. (d) The absorption spectra for pristine BPQD (i) and zigzag reconstructed edge BPQD with hydrogen passivation (iii-H).

studies.34−37 These optimized PD BPQDs remain planar except that interstitial defect type has slight distortion. Careful

viii)). These defects have been widely studied in BP nanosheets and nanoribbons in earlier experimental or theoretical 162

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zigzag and armchair edge of partial H passivation BPQD, in which P atoms are coordinated by 2, fully H passivation will eliminate the dangling bonds of P atoms and turn them into pristine BPQD. For the interstitial or zigzag edge reconstruction defective BPQD with CN = 4 or 5 P atoms, the excess H atoms are prone to dissociate when ventilated with hydrogen, and the saturated P bonds are thus obtained. Careful orbital analysis shows that the wave function isosurfaces of HOMO and LUMO in pristine BPQD mainly concentrates in the center region, while those of zigzag reconstructed edge BPQD completely localize in the defective region (see Figure 2c). Upon ventilation with hydrogen, the HOMO and LUMO are localized at the center region and the IGSs then disappear, and thereby the quenched absorption spectra are well recovered (see Figure 2d). Now, we have a clear picture of the influence of intrinsic PDs on the electronic structure and optical absorption of BPQDs. There exist two types of photoabsorption depending on the CN of P atoms. Specifically, the saturated P atoms with CN = 3 always show excellent photoabsorption, while the unsaturated or oversaturated P atoms with CN ≠ 3 create in-gap-states (IGSs) and fully eliminate the photoabsorption. However, these IGSs can be easily removed via sufficient hydrogen passivation, and the photoabsorption is then repaired (see Figure 3). Therefore, we can conclude BPQDs have good photoabsorption tolerance to the intrinsic PDs.

examination shows that these PD BPQDs can be categorized into two types: The P atoms at the defect region are saturated with CN = 3 (type I), and the P atoms are unsaturated or oversaturated with CN ≠ 3 (type II). More specifically, the divacancy, armchair reconstructed edge and interstitial with H passivation PDs belong to type I (highlighted by magnified green in Figure 1a), while the zigzag reconstructed edge, interstitial, zigzag or armchair edge with partial passivation PDs are type II (highlighted by magnified red in Figure 1a) with CN = 2, 4, 5, respectively. The electronic gap of BPQDs, defined as the energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), i.e., the HOMO−LUMO gap, is depicted in Figure 1b. A closer examination shows there are two kinds of responses toward the presence of defects: the electronic gap of type I BPQDs (i.e., divacancy, armchair reconstructed edge and interstitial with H passivation) is comparable to that of the pristine BPQDs, while that of type II is reduced dramatically (the zigzag reconstructed edge, interstitial, zigzag edge and armchair edge with partial H passivation). The discrepancy arises from their different bonding nature. For type I BPQDs, as the P atoms are all saturated, the bonding nature therein is similar to that of pristine BPQD. While for type II BPQDs, the defective P atoms are 2, 4, 5-coordinate, the unsaturated or oversaturated P atoms give rise to IGSs in the electronic gap as illustrated in Figure 2a. Note that this is different from the monolayer BN and metal dichalcogenides (MX2), which their IGSs are mostly originated from the chemical disorder introduced by the homoelemental bonds (M−M, X−X, B−B, or N−N).38,39 Regarding of the optical properties, as displayed in Figure 1c, the pristine BPQD exhibits striking photoabsorption in visible light region. The main absorption is dominated by transition from the ground state S0 to the first excited singlet state S1 (see Figure S1). The lowest singlet transition is mostly between the HOMO and LUMO orbitals that are mainly concentrated in the central region of BPQD (see Figure 2c). The presence of PD defects induces significantly different absorption spectra. For the type I BPQDs, the absorption spectrum is greatly broadened, and it is blue-shifted upon the presence of divacancy or interstitial defects with H passivation. The broadened and blue-shifted absorption spectra are derived from the fact that their HOMO and LUMO orbitals are partly localized in the defective region which activates high transitions such as S0 → S2 and S0 → S3 (see Figure S1,2,3). Regarding the case of armchair reconstructed edge defect, it owns similar frontier orbitals to pristine BPQD (see Figure S4), and the S0 → S1 transition remains bright excitation; Nevertheless, the reduced electronic gap leads to the red-shift of the absorption spectrum compared with the pristine BPQDs. For the type II defects, the absorption spectra are completely quenched (see Figure S5). This is because the unsaturated or oversaturated P atoms in type II BPQDs create IGSs in the bandgap and localize the frontier orbitals (see Figure S6−S8), which acts as undesirable blockage for electron excitation.40 Since the unsaturated or oversaturated P atoms in defective BPQDs cause the IGSs and quench the absorption spectra, it is natural to ask whether it is possible to remove these IGSs and recover the absorption spectra via certain treatment? The answer is positive. The IGSs in defective BPQDs can be removed easily by adding additional hydrogen atoms, i.e., further hydrogen passivation. As shown in Figure 2b, for the

Figure 3. Correlation between structure-electronic and gap-absorption in BPQDs with PDs. Type I BPQDs with P CN = 3 show good optical absorption, while the unsaturated or oversaturated P atoms with CN ≠ 3 create IGSs and quench the absorption. Interestingly, type II defects can transform into type I under the circumstance of hydrogen, and the absorption spectrum is well recovered.

Oxidation Defects in BPQDs. We constructed seven different configurations in Figure 4a, which can also be categorized into two types according to the bonding nature of P atoms. The type I BPQDs include horizontal oxygen bridge at zigzag edge, vertical oxygen bridge at armchair edge, dangling oxygen at face, interstitial oxygen bridge at surface or interlayer, with P CN = 3. The type II is formed with P CN ≠ 3 due to the unsaturated bond between phosphorus and oxygen for BPQDs with dangling oxygen at zigzag edge or at armchair edge. As shown in Figure 4b, the electronic gap of type II ODs is decreased dramatically because the pz-orbitals of edge P and O atoms form weak unsaturated P−O bonds, which induce the IGSs, as depicted in Figure S9−S11. For type I ODs, the electronic gap of oxygen bridge at zigzag or armchair edge defect is reduced, while the other ODs increase the electronic gap slightly, compared with the pristine BPQD. The discrepancy arises from their different bonding strength. Although the bridge-bonded O atom in zigzag or armchair 163

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Figure 4. (a) Top and side views of optimized OD BPQDs. (i) dangling oxygen at zigzag edge, (ii) dangling oxygen at armchair edge, (iii) horizontal oxygen bridge at zigzag edge, (iv) vertical oxygen bridge at armchair edge, (v) dangling oxygen at face, (vi) interstitial oxygen bridge at face, and (vii) interstitial oxygen bridge at interlayer. P and H atoms are colored in orange and white, respectively. The O atoms forming saturated (unsaturated) bond with P are highlighted by magnified blue (red) ball, corresponding to type I (type II) ODs. (b) The electronic gap of OD BPQDs. (c) Absorption spectra of pristine or type I OD BPQDs. (d) Illustration of the vertical absorption excitation for the OD BPQDs. S0, S1, and Sn denote the ground state, the first singlet excited state and the nth high-lying singlet excited state, respectively. For the sake of clarity, we only marked bright excited states as red lines.

S17, the poor absorption of dangling oxygen at zigzag or armchair edge is well repaired. In summary, we systematically investigate the influence of PDs and ODs on the electronic and optical properties of BPQDs using TD-DFT. Our calculations reveal that BPQDs with PDs or ODs maintain excellent optical absorption features as long as the defective P atoms are saturated with CN = 3, and the optical absorption region is broadened. However, BPQDs containing unsaturated or oversaturated P atoms with CN ≠ 3 will completely destroy the optical absorption due to the introduction of the IGSs. Nevertheless, these IGSs can be eliminated easily by sufficient hydrogen passivation, which repairs the optical absorption of BPQDs. The defect optical tolerance enables BPQDs to serve as promising candidates for electronic and optoelectronic applications.

edge forms saturated bonds with two edge P atoms, two P−O bonds are elongated slightly, i.e., 1.76 and 1.76 Å for zigzag and 1.71 and 1.76 Å for armchair edge, respectively. The elongated P−O bonds induce edge states close to the HOMO for zigzag edge or LUMO for armchair edge defect, leading to the slightly reduced gap (see Figure S9, S12, S13), whereas, for the dangling oxygen atom at surface, the lone pair electron of P atom is attracted by oxygen atom due to large electronegative, forming the strong and polar saturated bond with a bond length of 1.51 Å. In the case of interstitial oxygen bridge at surface and interstitial oxygen bridge at interlayer, the oxygen atom penetrates into the lattice occupying the center position of the P−P bond, forming a stable bridge bond with two P atoms. The last three OD types (the dangling oxygen atom at surface, interstitial oxygen bridge at surface, and interstitial oxygen bridge at interlayer) tend to enlarge the electronic gap and frontier orbitals are delocalized (see Figure S14−S16), which was also observed in 2D phosphorene.41 Similar to the cases of PDs, there are two kinds of photoabsorption response in oxidized BPQDs. As shown in Figure 4c, the type I ODs possess excellent optical absorption in visible light region. The ODs can further tune and broaden photoabsorption region of BPQDs. The horizontal oxygen bridge at zigzag edge defect leads to the red-shift of the absorption spectrum, while the dangling oxygen at surface and interstitial oxygen bridge at surface or interlayer defects cause blue-shift of the absorption region. It is worth noting that the ODs increase the number of bright transitions compared with the only one S0 → S1 bright transition in pristine BPQD as shown in Figure 4d. However, the type II ODs are detrimental to optical absorption due to the existence of IGSs. Similar to the case of PDs, when these ODs are passivated by hydrogen, the pz-orbitals of P and O atoms form saturated P−O bond, which successfully eliminates the IGSs. As displayed in Figure

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b02486. Illustration of the vertical absorption excitation for the pristine and PD BPQDs (Figures S1,5), DOS of BPQDs and OD BPQDs (Figure S9), the frontier orbitals of PD and OD BPQDs (Figures S2−4,6−8,10−16), absorption spectra of repaired hydroxyl termination in armchair or zigzag edge of BPQDs (Figure S17) (PDF)

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AUTHOR INFORMATION

ORCID

Xianghong Niu: 0000-0001-6475-8839 Huabing Shu: 0000-0001-9278-185X Yunhai Li: 0000-0003-4692-8114 Jinlan Wang: 0000-0002-4529-874X 164

DOI: 10.1021/acs.jpclett.6b02486 J. Phys. Chem. Lett. 2017, 8, 161−166

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The Journal of Physical Chemistry Letters Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the NSFC (21525311, 21373045) and SRFDP (20130092110029) in China, the Scientific Research Foundation of Graduate School of Southeast University (YBJJ1620), and Jiangsu Innovation Projects for Graduate Students (KYZZ16_0117) in China. The authors thank the computational resources provided by Southeast University.

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