Carrier recombination spatial transfer by reduced potential barrier causes blue/red switchable luminescence in C8 carbon quantum dots/organic hybrid light emitting devices Carrier recombination spatia[.]
Carrier recombination spatial transfer by reduced potential barrier causes blue/red switchable luminescence in C8 carbon quantum dots/organic hybrid light-emitting devices Xifang Chen, Ruolin Yan, Wenxia Zhang, and Jiyang Fan Citation: APL Mater 4, 046102 (2016); doi: 10.1063/1.4945722 View online: http://dx.doi.org/10.1063/1.4945722 View Table of Contents: http://aip.scitation.org/toc/apm/4/4 Published by the American Institute of Physics Articles you may be interested in Optimal nitrogen and phosphorus codoping carbon dots towards white light-emitting device APL Mater 109, 083103083103 (2016); 10.1063/1.4961631 Optical spectroscopy reveals transition of CuInS2/ZnS to CuxZn1-xInS2/ZnS:Cu alloyed quantum dots with resultant double-defect luminescence APL Mater 4, 126101126101 (2016); 10.1063/1.4971353 APL MATERIALS 4, 046102 (2016) Carrier recombination spatial transfer by reduced potential barrier causes blue/red switchable luminescence in C8 carbon quantum dots/organic hybrid light-emitting devices Xifang Chen, Ruolin Yan, Wenxia Zhang, and Jiyang Fana Department of Physics and Jiangsu Key Laboratory for Advanced Metallic Materials, Southeast University, Nanjing 211189, People’s Republic of China (Received 11 March 2016; accepted 29 March 2016; published online April 2016) The underlying mechanism behind the blue/red color-switchable luminescence in the C8 carbon quantum dots (CQDs)/organic hybrid light-emitting devices (LEDs) is investigated The study shows that the increasing bias alters the energy-level spatial distribution and reduces the carrier potential barrier at the CQDs/organic layer interface, resulting in transition of the carrier transport mechanism from quantum tunneling to direct injection This causes spatial shift of carrier recombination from the organic layer to the CQDs layer with resultant transition of electroluminescence from blue to red By contrast, the pure CQDs-based LED exhibits green–red electroluminescence stemming from recombination of injected carriers in the CQDs C 2016 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4945722] Colloidal quantum dots (QDs)-based light-emitting devices (LEDs) have attracted great interest in recent years due to their wide applications in flat-panel display, solid-state lighting, and optical communication, etc.1 Moreover, the continuous and smooth semiconductor QDs films used in the LEDs are readily to fabricate by using simple solution-processed deposition technique.2 Since the first report on CdSe QD-LED in 1994,3 various types of QDs such as CdSe/CdS, Cd1−xZnxSe1−ySy, ZnCdS/ZnS, PbS/CdS, ZnO, and Si QDs have been employed to fabricate QD-LEDs.4–10 QDs have also been used as color converters in white LEDs.11,12 However, there have been very few studies focusing on carrier transport and recombination characteristics of QD-LEDs On the other hand, compared with the wurtzite or zinc blende structured Cd-containing QDs, heavy-metal-free carbon QDs (CQDs) are more benign to human beings and the environment.13,14 They have become a research focus owing to low cytotoxicity, exploited various synthesis methods, unexampled abundance of source materials on Earth, and robust near-infrared to near-UV luminescence.15,16 CQDs usually exist as nanodiamonds or graphite/graphene quantum dots.17 We have recently synthesized luminescent C8 (third carbon allotrope) CQDs with sizes ranging from about 3.5 nm down to below nm.18 Here, we design and fabricate both pure and inorganic/organic hybrid LEDs based on C8 CQDs and investigate their unusual carrier transport/recombination and electroluminescence (EL) properties The first-type designed device (named device A) is a pure CQDs-based LED It has a simple structure without carrier transport layers such that it can accommodate pure luminescence from the CQDs By contrast, the second-type device (named device B) contains carrier transport layers to ensure higher quantum efficiency and more device stability.19,20 Previous study has indicated that the excimers and electromers are prone to form in organic hole-transport materials and their light emissions enlarge the EL spectral region.21,22 Although their light emissions are weak in usual semiconductor QD-LEDs due to energy transfer,23,24 but their existence makes it hard to discriminate the contribution of the QDs to electroluminescence of the conventional-structured a Author to whom correspondence should be addressed Electronic mail: jyfan@seu.edu.cn 2166-532X/2016/4(4)/046102/9 4, 046102-1 © Author(s) 2016 046102-2 Chen et al APL Mater 4, 046102 (2016) LEDs The current device A has a three-layer structure with the following order: indium tin oxide (ITO), 100-nm-thick C8 CQDs, and Al film (150 nm) Device B has a complex structure with the following order: ITO, CQDs embedded in poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS:CQDs, 70 nm), 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl) benzene (TPBI, 12 nm), and LiF/Al (3/150 nm) The excimer and electromer emissions are effectively prohibited in both devices The PEDOT:PSS acts as the transparent hole-injection layer.25,26 The TPBI film acts as the electron-transport layer The ITO film acts as the anode and the Al and LiF/Al films act as the cathode The ITO was deposited on a glass substrate by using the magnetron sputtering method It has a thickness of 135 nm, a sheet resistance of 10 Ω/sq, and a light transmittance of over 80% The CQDS and PEDOT:PSS:CQDs films were prepared by sol-gel spin-cast All the spin-coated films were subsequently annealed in air at 110 ◦C for 30 to improve uniformity and stability The heating temperature for PEDOT:PSS is usually above 110 ◦C We adopted the lower temperature in our experiment considering thermal stability of the CQDs The thickness of the spin-coated film is controlled by altering the CQDs concentration and the spinning speed For fabrication of device A, the spinning speed was chosen as 2000 rpm It is found that device A fabricated with 30 kg/m3 CQDs solution (100-nm-thick CQDs layer) exhibits the best electroluminescence performance For fabrication of device B, the spinning speed is chosen as 2500 rpm, and the concentration of the (PEDOT:PSS + CQDs) solution was selected as 12 kg/m3 The Al cathode in device A as well as the TPBI film and LiF/Al cathode in device B was separately deposited with help of a shadow mask in high vacuum of × 10−6 Torr by using a thermal-evaporation system The evaporation rate was 0.1 Å/s for LiF and TPBI films and 4–5 Å/s for Al film The active area of the device was × mm2 as defined by the overlapped area of the ITO and Al electrodes The devices were not encapsulated and all the measurements were performed in air The atomic force microscopy (AFM) images and the thickness of the films were acquired by using a Dimension Icon atomic force microscope (Bruker Corporation) with resolution of nm along X and Y axes and resolution of 0.05–0.1 nm along Z axis The photoluminescence (PL) spectra were measured by using a Fluorolog 3-TCSPC spectrofluorometer (HORIBA JOBIN YVON) with a xenon lamp as the light source The electroluminescence spectra were measured by using an Acton SP-2358 Spectrometer (Princeton Instruments) The electrical characterization of the devices was performed by using a Keithley 2400 source meter The AFM images (Fig 1) reveal that the CQDs film spin-coated on the ITO substrate (device A) is smooth with a root mean square (RMS) roughness of 0.30 nm This value is close to that of the monolayer of close packed ZnCdSe alloyed QDs (0.4 nm) used in the QD-LED.27 The PEDOT:PSS:CQDs layer on ITO in device B has a larger RMS roughness of 5.08 nm The AFM images suggest that there is no phase separation for this layer, revealing that the CQDs are embedded in the PEDOT:PSS matrix Figure 2(a) shows the PL spectra of the CQDs dispersed in water As can be seen, the PL peak shifts from 520 to 620 nm as the excitation wavelength increases from 420 to 580 nm The excitation dependence of the PL peak is not related to the quantum confinement effect because bulk C8 carbon is an insulator with an indirect bandgap of 5.5 eV and a direct bandgap ranging from 6.5 to 11 eV,28 the probable luminescence stemming from interband transition of carriers in C8 carbon should lie in the ultraviolet region29 rather than in the current visible region The origin of the photoluminescence of the CQDs dispersed in water had been investigated in detail by using various characterizations (PL, UV-vis absorption, infrared absorption, X-ray photoelectron spectroscopy) in our recent report.18 The combined characterizations demonstrate that electron transition between the surface states associated with the C(==O)O functional groups generates the photoluminescence The addition of other chemical groups in the adjacent of the C(==O)O group usually makes the PL peak shift to red In particular, additional conjugation of the C==C double bond to the C(==O)O group causes considerable red shift of over 30 nm Hence, there are several separated emission peaks in the visible region resulting from different C(==O)O-related surface states The total PL spectrum resulting from their superimposition shows red shift with increasing excitation wavelength The C8 CQD-LEDs exhibit interesting electroluminescence properties Figure 2(b) shows the EL spectra of device A under different biases The EL spectral region ranges from 400 to 900 nm The maximum of the EL spectrum lies at around 650 nm, and the intensity of the maximum 046102-3 Chen et al APL Mater 4, 046102 (2016) FIG Two-dimensional and three-dimensional AFM images of ((a) and (b)) close-packed CQDs film and ((c) and (d)) PEDOT:PSS:CQDs composite film spin-coated on ITO substrates increases first and decreases then with increasing bias The EL spectrum has a full width at half maximum (FWHM) of about 250 nm, which is much larger than that of the PL spectrum (about 100 nm) [Fig 2(a)] This is because that the injected carriers can occupy much more energy levels in the case of EL compared with the photon-excited electrons in the case of PL The energy-level distribution of the injected carriers determines the line shape and maximum position of the EL spectrum The EL characteristics of device B are quite different from that of device A The features of the EL spectra of device B for bias 4–8 V [Fig 2(c)] resemble that of the PL spectra of the TPBI film [Fig 2(a)], and they have the same peak position at around 390 nm and nearly equal linewidths These features prove that the violet EL stems from recombination of electrons and holes in the TPBI layer It becomes most intense at V and then diminishes gradually with further increasing bias The EL spectral features totally change for bias >9 V [Fig 2(d)] On the one hand, the violet emission from TPBI vanishes gradually with increasing bias On the other hand, a broad EL band lying in the green–red region arises, and it becomes more intense with increasing bias The measurement indicates that the pure PEDOT:PSS layer has no electroluminescence, hence, the green–red region EL must originate from the CQDs In practice, as far as the spectral region and linewidth are concerned, the green–red EL spectrum roughly resembles the EL spectrum of device A that arises from the CQDs The conversion of the electroluminescence band from blue region to green–red region with increasing bias in device B suggests that the recombination space of the injected carriers transfers from the TPBI layer to the PEDOT:PSS:CQDs layer As can be seen from Fig 2(d), for bias >12 V, there is only green–red EL band, suggesting under this circumstance, the injected electrons and holes recombine completely in the PEDOT:PSS:CQDs layer The characteristics of the three EL emission bands (red–green band from CQDs in device A, blue band from TPBI, and red–green band from CQDs embedded in PEDOT:PSS in device B) are more clearly embodied by the curves of variation of the peak wavelength and intensity of these 046102-4 Chen et al APL Mater 4, 046102 (2016) FIG (a) Photoluminescence spectra of spin-coated TPBI film and CQDs in water (b) Electroluminescence (EL) spectra of device A under different driving biases (c) EL spectra of device B under (c) low bias [inset: EL image at V] and (d) high bias [inset: EL image at 10 V] emission bands with bias, as shown in Fig Both the peak wavelength and intensity show different variation trends with bias for three emission bands For device A, the EL peak wavelength is nearly independent of bias, and the EL peak intensity versus bias curve is very narrow, both features are characteristic of the luminescence of an organic film In contrast, the red–green EL bands from both devices vary dramatically with bias, suggesting that there are considerable lower and higher energy levels involved in electroluminescence of the CQDs This is because, on the one hand, the CQDs have a loose size distribution, the conduction band minimum (relative to vacuum energy level) increases and the valence band maximum decreases with decreasing particle size due to the quantum confinement effect; on the other hand, there are several types of C(==O)O-related surface FIG (a) EL spectrum peak wavelength versus bias (b) EL spectrum peak intensity versus bias 046102-5 Chen et al APL Mater 4, 046102 (2016) states in the CQDs and they have different energy levels Although the red–green region EL from both devices stems from the CQDs, however, they show different variation trends with bias, such a difference suggests that there are distinct energy-level spatial distributions and carrier transport and recombination mechanisms in these two devices of different spatial structures There are some measures to improve the electroluminescence intensity of the devices for practical applications For device A, the weak EL may be ascribed to the high potential barriers for carrier injection, especially the barrier for electrons at the CQDs-Al interface.30 Another reason lies in the fact that direct contact between the light emission layer and the electrodes leads to high rate of nonradiative Auger recombination at the interface.31 For device B, the energy transfer between the CQDs and the adjacent polymer layers may result in significant diminishing of EL The quantum efficiency of the CQDs is another limitation factor Hence, suppressing the energy transfer efficiency and improving the quantum yield of the CQDs will lead to enhanced EL The multilayer structure may also affect the emission efficiency of the CQDs and optimized device structure with more favorable film quality is helpful The spotty EL image of the devices (insets of Fig 2) suggests nonuniformity of EL owing to nonuniform films resulting from irregular interfaces between domains of the same layer and irregular interfaces between different layers The EL image in inset of Fig 2(d) contains a lighter emission band on the left-hand side because this edge area is more uniform as a result of shadow-mask covering during deposition of TPBI, LiF, and Al films Therefore, improvement of film uniformity will result in more uniform EL of the device In order to reveal the underlying physics for the respective electroluminescence spectral features in pure and hybrid CQDs-based LEDs, especially for the strange and rare phenomenon of electroluminescence-color switching in device B, it is necessary to analyze the energy-level spatial distribution and the carrier transport and recombination mechanisms in these devices The left (top and bottom) panel in Fig shows schematic energy-level diagrams of two CQDs-based LEDs The work functions of the ITO anode and the Al cathode are 4.7 and 4.3 eV (below vacuum energy level), respectively.32 The PEDOT:PSS film improves the work function from 4.3 eV (ITO) to 5.0 eV.33,34 The values of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of TPBI are separately 2.8 and 6.3 eV The CQDs layer possesses at least two types of surface-state energy levels generated by different ester-related surface functional groups.18 Figure displays the measured current density versus bias (J–V ) curves for both devices, and the semi-logarithmic coordinates are employed (the insets show the J–V curves with linear coordinates) The circled dots and solid lines correspond to the experimental data and fitted curves, respectively The different regions of J–V data are well fitted by using different equations and there is no any single equation that can fit the whole region of J–V data This suggests that the carrier transport and recombination mechanisms change with increasing bias in the same device Various models have been proposed to explain carrier transport properties in (especially organic) LEDs, including Fowler-Nordheim tunneling,35 Poole-Frenkel emission,36 trap-limited transport,37 and space charge-limited current38 models However, these models cannot account for the J–V characteristics of the current CQDs-based LEDs For device A, the J–V data for bias 4 V is governed by direct electron injection from the cathode and direct hole injection from the anode (Fig 4: top right) These injected electrons and holes encounter in the CQDs and recombine therein to emit photons of green–red spectral region We now explain the more complex electroluminescence phenomenon in device B by analyzing the energy-level spatial distribution and associated carrier transport and recombination picture In device B, the potential barrier for holes to go into the CQDs layer from ITO anode is reduced by 0.3 eV owing to introduction of a intermediate potential-barrier step ascribed to the added PEDOT:PSS layer (Fig 4: bottom left) This change is favorable for direct injection of holes into the CQDs layer and the following TPBI layer under driving voltage, and these advancing holes will recombine with the electrons at the TPBI/LiF-Al interface (near the Al layer) injected from the cathode This explains why the J–V data under bias 11 V, both the electron potential barrier at the interface of the TPBI and PEDOT:PSS:CQDs layers and the electron potential barrier at the TPBI/Al interface cease gradually (Fig 4: bottom right), this is because that these two potential barrier spatial regions have the highest electric resistance and thus the applied voltage mainly falls in these two thin regions, thereby significantly reducing the barrier height Hence, the electron transport is dominated by direct injection across the whole device and the current density increases rapidly with bias This scenario corresponds to the case of zone III in Fig 5(b) Indeed, in this region, the J–V data are well fitted by Eq (3) Under this circumstance, the electrons and holes recombine only in the CQDs, and this explains why there is only green–red electroluminescence in zone III Because the current density is very large in the mode of direct injection of electrons, so the electroluminescence intensity is highly improved in this region [Fig 2(d)] It should be noted that the electroluminescence performance of device B is sensitive to the thickness of the TPBI layer The turn-on voltage increases from to V as the thickness of 046102-8 Chen et al APL Mater 4, 046102 (2016) the TPBI layer increases from 12 to 16 nm, meanwhile, the electroluminescence intensity decreases remarkably Such turn-on voltage is comparable to usual QD-LEDs and can be further reduced by using thinner TPBI layer; however, if the TPBI layer is too thin, the EL intensity will decrease and the normal-operation voltage range will become narrower Hence, there is an optimal thickness of TPBI film The above discussions indicate that the operation mechanisms of the QDs-based LEDs can be revealed based on combined study of the electroluminescence spectra and the J–V curves of the LEDs in conjunction with analysis of the bias dependence of the energy-level spatial distribution These results also show that electroluminescence with tunable colors can be realized by using a single QDs-based LED device with intentionally designed multilayered structure in which there are multiple carrier potential barriers and thus the recombination spatial region of electrons and holes is transferable and controlled by varying the driving voltage In summary, we have observed intriguing and peculiar phenomenon of blue/red switchable electroluminescence in inorganic/organic hybrid LED based on C8 CQDs A complete model concerning carrier recombination spatial transfer is proposed to explain the experimental phenomenon based on analysis of the carrier transport and recombination mechanisms It was found that quantum tunneling and direct injection alternately dominate the carrier transport mechanism as the bias increases and this leads to transferable carrier recombination spatial region The pure CQDs-based LED exhibits totally green–red electroluminescence originating from the CQDs These findings largely improve our understanding of the electroluminescence mechanisms of the QDs-based LEDs This work was supported by the National Natural Science Foundation of China Nos 11274063 and 11574047 It is also supported by Jiangsu Key Laboratory for Advanced Metallic Materials (BM2007204) Y Shirasaki, G J Supran, M G Bawendi, and V Bulovi´c, Nat Photonics 7, 13 (2013) H Dondapati, D Ha, and A K Pradhan, Appl Phys Lett 103, 121114 (2013) V L Colvin, M C Schlamp, and A P Alivisatos, Nature 370, 354 (1994) X Dai, Z Zhang, Y Jin, Y Niu, H Cao, X Liang, L Chen, J Wang, and X Peng, Nature 515, 96 (2014) Y Yang, Y Zheng, W Cao, A Titov, J Hyvonen, J R Manders, J Xue, P H Holloway, and L Qian, Nat Photonics 9, 259 (2015) H Huang, C Dun, W Huang, Y Cui, J Xu, Q Jiang, C Xu, H Zhang, S Wen, and D L Carroll, Appl Phys Lett 106, 263506 (2015) Y Z Xin, K Nishio, and K Saitow, Appl Phys Lett 106, 201102 (2015) B N Pal, Y Ghosh, S Brovelli, R Laocharoensuk, V I Klimov, J A Hollingsworth, and H Htoon, Nano Lett 12, 331 (2012) H Shen, W Cao, N T Shewmon, C Yang, L S Li, and J Xue, Nano Lett 15, 1211 (2015) 10 G J Supran, K W Song, G W Hwang, R E Correa, J Scherer, E A Dauler, Y Shirasaki, M G Bawendi, and V Bulovi´ c, Adv Mater 27, 1437 (2015) 11 L Yang, Y Liu, Y.-L Zhong, X.-X Jiang, B Song, X.-Y Ji, Y.-Y Su, L.-S Liao, and Y He, Appl Phys Lett 106, 173109 (2015) 12 X T Feng, F Zhang, Y L Wang, Y Zhang, Y Z Yang, and X G Liu, Appl Phys Lett 107, 213102 (2015) 13 C Sun, Y Zhang, Y Wang, W Liu, S Kalytchuk, S V Kershaw, T Zhang, X Zhang, J Zhao, W W Yu, and A L Rogach, Appl Phys Lett 104, 261106 (2014) 14 X Y Xu, R Ray, Y l Gu, H J Ploehn, L Gearheart, K Raker, and W A Scrivens, J Am Chem Soc 126, 12736 (2004) 15 S N Baker and G A Baker, Angew Chem., Int Ed 49, 6726 (2010) 16 J Fan and P K Chu, Small 6, 2080 (2010) 17 L.-S Li and X Yan, J Phys Chem Lett 1, 2572 (2010) 18 X Chen, W Zhang, Q Wang, and J Fan, Carbon 79, 165 (2014) 19 B S Mashford, M Stevenson, Z Popovic, C Hamilton, Z Zhou, C Breen, J Steckel, V Bulovic, M Bawendi, S C Sullivan, and P T Kazlas, Nat Photonics 7, 407 (2013) 20 P O Anikeeva, C F Madigan, J E Halpert, M G Bawendi, and V Bulovi´ c, Phys Rev B 78, 085434 (2008) 21 J Kalinowski, M Cocchi, D Virgili, V Fattori, and J A G Williams, Adv Mater 19, 4000 (2007) 22 Z Zhao, B Xu, Z Yang, H Wang, X Wang, P Lu, and W Tian, J Phys Chem C 112, 8511 (2008) 23 V T Forster, Ann Phys 6, 55 (1948) 24 Y Li, A Rizzo, M Mazzeo, L Carbone, L Manna, R Cingolani, and G Gigli, J Appl Phys 97, 113501 (2005) 25 K van de Ruit, R I Cohen, D Bollen, T van Mol, R Yerushalmi-Rozen, R A J Janssen, and M Kemerink, Adv Funct Mater 23, 5778 (2013) 26 Y H Huang, W L Tsai, W K Lee, M Jiao, C Y Lu, C Y Lin, C Y Chen, and C C Wu, Adv Mater 27, 929 (2015) 27 P O Anikeeva, J E Halpert, M G Bawendi, and V Bulovi´ c, Nano Lett 9, 2532 (2009) 28 R L Johnston and R Hoffmann, J Am Chem Soc 111, 810 (1989) 29 P Liu, Y L Cao, C X Wang, X Y Chen, and G W Yang, Nano Lett 8, 2570 (2008) 046102-9 30 Chen et al APL Mater 4, 046102 (2016) S Yang and M Jiang, Chem Phys Lett 484, 54 (2009) W K Bae, Y S Park, J Lim, D Lee, L A Padilha, H McDaniel, I Robel, C Lee, J M Pietryga, and V I Klimov, Nat Commun 4, 2661 (2013) 32 W Ji, P Jing, W Xu, X Yuan, Y Wang, J Zhao, and A K Y Jen, Appl Phys Lett 103, 053106 (2013) 33 Q Sun, Y A Wang, L S Li, D Wang, T Zhu, J Xu, C Yang, and Y Li, Nat Photonics 1, 717 (2007) 34 X Yang, Y Divayana, D Zhao, K S Leck, F Lu, S T Tan, A P Abiyasa, Y Zhao, H V Demir, and X W Sun, Appl Phys Lett 101, 233110 (2012) 35 D Parker, J Appl Phys 75, 1656 (1994) 36 S Habermehl and C Carmignani, Appl Phys Lett 80, 261 (2002) 37 P E Burrows and S R Forrest, Appl Phys Lett 64, 2285 (1994) 38 F Torricelli, D Zappa, and L Colalongo, Appl Phys Lett 96, 113304 (2010) 39 J Warga, R Li, S N Basu, and L Dal Negro, Appl Phys Lett 93, 151116 (2008) 40 Q Huang, S Zhao, Z Xu, X Fan, C Shen, and Q Yang, Appl Phys Lett 104, 161112 (2014) 41 B S Kim and J Y Lee, Org Electron 21, 100 (2015) 42 J Song, J Li, X Li, L Xu, Y Dong, and H Zeng, Adv Mater 27, 7162 (2015) 31 ... (2016) Carrier recombination spatial transfer by reduced potential barrier causes blue/red switchable luminescence in C8 carbon quantum dots/organic hybrid light- emitting devices Xifang Chen, Ruolin... observed intriguing and peculiar phenomenon of blue/red switchable electroluminescence in inorganic/organic hybrid LED based on C8 CQDs A complete model concerning carrier recombination spatial transfer. .. color -switchable luminescence in the C8 carbon quantum dots (CQDs)/organic hybrid light- emitting devices (LEDs) is investigated The study shows that the increasing bias alters the energy-level spatial distribution