您当前的位置: 首页 > 资源详细信息
资源基本信息 
来源机构: 《自然杂志》
来源目录: news
发布日期: 2021-12-4
资源类型: 188.78KB
资源性质: 重要新闻
重要度:   
资源评价:

资源推荐:

主题相关资源
监测目标主题
目标主题     
(1)
系统抽取主题
抽取主题     
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)

Mid-infrared irradiation keeps nanocrystals bright | Nature Nanotechnology

Mid-infrared pulses stimulate fast neutralization of photocharged colloidal nanocrystals, which suppresses blinking of a single nanocrystal’s photoluminescence. You have full access to this article via your institution. Download PDF Download PDF Since their discovery 40 years ago, semiconductor nanocrystal quantum dots or nanocrystals have become the most heavily studied nanoscale semiconductors that have high market demand 1 . However, an intrinsic instability of radiative emission from a single nanocrystal— a random alternation of on and off periods even under constant illumination, known as blinking 2 — limits potential applications. This undesirable blinking of the photoluminescence most often, although not always 2 , 3 , stems from spontaneous photocharging processes that interrupt the excitation–emission cycle, as shown schematically in Fig. 1 . The ionization process switches the nanocrystal from the neutral emissive state to the charge-separated state, with one of the carriers ejected and trapped outside the nanocrystal core. This process is reversible: sooner or later, the charge neutrality of the core is restored. However, for as long as the nanocrystal remains in the charged state, any excited electron–hole pair that recombines will transfer its energy to the excessive charge via the non-radiative three-particle Auger process, so that the nanocrystal does not emit light. Many efforts have been directed toward chemical synthesis and optimization of core–shell structures to prevent the spontaneous ionization of the nanocrystal core as well as to decrease the efficiency of the Auger recombination 2 . Now, writing in Nature Nanotechnology , Jiaojian Shi, Weiwei Sun and co-workers suggest a different solution to the problem of suppressing blinking: ultrashort (150 fs, 5.5 μm) mid-infrared (MIR) pulses can stimulate the neutralization of the nanocrystal core and thereby suppress blinking 4 . Fig. 1: Photocharging processes in colloidal semiconductor nanocrystals. Excitation by light creates an electron–hole pair (exciton), bringing nanocrystals from the empty state A to the emissive neutral state B. Radiative recombination of the exciton results in light emission. The excitation–emission cycle between A and B states can be interrupted spontaneously by the ionization of the nanocrystal: various ionization processes depicted by a single red arrow transfer the state B to the charge-separated state C, with one of the carriers left in the core (here, an electron) and the other one (here, a hole) ejected from the core and trapped outside at one of the trap states. The small blue and red circles depict the various electron-accepting (positive) and hole-accepting (negative) traps outside the core. The spontaneous reverse processes restore the charge neutrality of the core. Three neutralization pathways are shown by blue arrows: C → B, with the characteristic time τ return , for the return of the trapped carrier to the core; C → A with τ rec for the direct recombination of the core and surface-trapped carriers; and C → D with τ trap for the trapping of the core carrier at the surface. The C → B return is followed by delayed emission, whereas the transition C → D can be followed by surface recombination. Straight and wavy arrows indicate non-radiative and radiative processes, respectively. MIR pulses (wavy orange arrow) stimulate the neutralization process, for example C → D (straight orange arrow) with characteristic time τ MIR , so that the total lifetime of the charged nanocrystal state τ charged = ( τ return ?1 + τ rec ?1 + τ trap ?1 + τ MIR ?1 ) ?1 is considerably shortened. Full size image Back in 1996, the lead author of ref. 4 , Moungi Badwendi, together with fellow researchers, originally reported photoluminescence blinking in single nanocrystals 5 . Before the discovery of this blinking at the single-nanocrystal level, the reversible photoinduced ionization of semiconductor nanocrystals had been recognized as the reason for the photoluminescence degradation — a decrease of the photoluminescent intensity with time under constant illumination, in ensemble measurements 6 . There is a variety of ionization channels resulting in negative (as shown in Fig. 1 ) or positive photocharging: thermo-ejection; direct tunnelling of the carriers to the trapped states outside the nanocrystal core; or Auger-driven ionization involving an extra charge from the second excited electron–hole pair 2 or from the trap state 7 . There are also different ways for neutralization to occur; the most obvious is the release of the ejected carrier from the trap back to the nanocrystal core via tunnelling or thermo-activation. If the carrier returns to the core fast enough, it is hard to detect the blinking of a single nanocrystal. However, delayed emission from the nanocrystal core detected at times much larger than the exciton lifetime 8 provided evidence that the excitation–emission cycle was interrupted by the temporal off-state and proved the return of the trapped carrier to the core. The release of the trapped carrier that restores the neutral exciton emission can be thermo-stimulated 6 , 9 . In the new work, instead of heating, the researchers use moderate-electric-field MIR pulses to affect the photocharged state in CdSe/CdS nanocrystals 4 . They demonstrate that in nanocrystals with eight-monolayer CdS shells, the ultrafast pulses suppress the off periods of blinking. The effect is explained as field-stimulated tunnelling of the excessive charge from the core to some surface trap. In nanocrystals with much thicker, 14-monolayer CdS shells, the MIR pulses produce the opposite effect: they move the ejected carrier from the reversible to the irreversible trapped state outside the nanocrystal core, hinder the neutralization and thereby suppress the delayed emission from the core. As in nearly all breakthrough observations, the demonstrated effects raise new questions and call for further experimental and theoretical studies. Is the non-reversibility of the neutralization supported by a subsequent surface recombination? Can field-induced tunnelling of the trapped carrier from the surface back to the nanocrystal core occur in these or other structures? Can the field-induced tunnelling stimulate direct recombination of the core and the surface-trapped carriers? On a long timescale, what happens to the charge carriers ejected and removed far from the core? What is the effect on the electronic states inside the nanocrystals of the electric fields created by the charge distribution outside the cores? Can the MIR pulses result in just local heating of the nanocrystal? How will the field-induced effects depend on the temperature? What other sources of non-invasive nanocrystal excitation can be used? Independent of the particular microscopic mechanism, the non-invasive MIR-stimulated neutralization can stabilize emission and increase emission efficiency in existing applications of semiconductor and perovskite nanocrystals as well as in new applications of nanostructures that exhibit photo-ionization processes. In structures with suppressed Auger recombination, a different effect occurs: a random alternation of the emission times due to the radiative recombination of neutral excitons and negative trions 10 . One can expect MIR pulses to keep the nanocrystal neutral and thus suppress this ‘lifetime blinking’. In the alternative scheme, MIR pulses could intentionally stabilize the photocharging process instead of stimulating nanocrystal neutralization. Such a stabilization will allow permanently charged nanocrystals to be achieved without chemical doping. Long-lived negative photocharging of the giant-shell CdSe/CdS nanocrystals in vacuum was originally demonstrated in ref. 11 . On suppression of the Auger recombination, such permanently photocharged nanocrystals exhibit bright and stable trion emission. Charged nanocrystals can be used in applications that are based on the spin degree of freedom. The findings of Shi and co-workers offer new tools for the control of photocharging dynamics and the study of fundamental processes in ensembles of semiconductor or perovskite nanocrystals that are affected by the photocharging. For example, it was recently demonstrated that the long-time dynamics of the optically driven polarization of surface-localized spins correlates with the dynamics of the photocharging in the ensemble of bare-core CdSe nanocrystals 12 . Pump–probe Faraday rotation experiments with pre-pump-stimulated photo-ionization allowed researchers both to study the spin dynamics and to trace a spontaneous evolution from negative to positive photocharging in the ensemble of CdS nanocrystals 13 . The unpaired carriers inside the nanocrystals were spin-polarized by pumping with circular-polarized light. Incorporation of MIR pulses into spin and polarization dynamics experiments is a way to neutralize the nanocrystals and thus erase the prepared spin polarization on demand. Such control over the charged and neutral states will shed light on the microscopic origin of the blinking phenomenon and allow us to develop a new class of non-blinking nanocrystals. References . 1. Efros, A. L. & Brus, L. E. ACS Nano 15 , 6192–6210 (2021). CAS ? Article ? Google Scholar ? 2. Efros, A. L. & Nesbitt, D. J. Nat. Nanotechnol. 11 , 661–671 (2016). CAS ? Article ? Google Scholar ? 3. Galland, C. et al. Nature 479 , 203–207 (2011). CAS ? Article ? Google Scholar ? 4. Shi, J. et al. Nat. Nanotechnol . https://doi.org/10.1038/s41565-021-01016-w (2021). 5. Nirmal, M. et al. Nature 383 , 802–804 (1996). CAS ? Article ? Google Scholar ? 6. Grabovskis, V. Y. A. et al. Sov. Phys. Solid State 31 , 149–151 (1989). Google Scholar ? 7. Jain, A., Voznyy, O., Korkusinski, M., Hawrylak, P. & Sargent, E. H. J. Phys. Chem. Lett. 8 , 3179–3184 (2017). CAS ? Article ? Google Scholar ? 8. Ryabouw, F. T. et al. Nano Lett. 15 , 7718–7725 (2015). Article ? Google Scholar ? 9. Shornikova, E. V. et al. Nano Lett. 20 , 1370–1377 (2020). CAS ? Article ? Google Scholar ? 10. Galland, C. et al. Nat. Commun. 3 , 908 (2012). Article ? Google Scholar ? 11. Javaux, C. et al. Nat. Nanotechnol. 8 , 206–212 (2013). CAS ? Article ? Google Scholar ? 12. Biadala, L. et al. Nat. Nanotechnol. 12 , 569–575 (2017). CAS ? Article ? Google Scholar ? 13. Feng, D. et al. Nano Lett. 17 , 2844–2851 (2017). CAS ? Article ? Google Scholar ? Download references Author information . Affiliations . Ioffe Institute, Russian Academy of Sciences, St Petersburg, Russia Anna V. Rodina Authors Anna V. Rodina View author publications You can also search for this author in PubMed ? Google Scholar Corresponding author . Correspondence to Anna V. Rodina . Ethics declarations . Competing interests . The author declares no competing interests. Rights and permissions . Reprints and Permissions About this article . Cite this article . Rodina, A.V. Mid-infrared irradiation keeps nanocrystals bright. Nat. Nanotechnol. (2021). https://doi.org/10.1038/s41565-021-01029-5 Download citation Published : 03 December 2021 DOI : https://doi.org/10.1038/s41565-021-01029-5 Share this article . Anyone you share the following link with will be able to read this content: Get shareable link Sorry, a shareable link is not currently available for this article. Copy to clipboard Provided by the Springer Nature SharedIt content-sharing initiative .

原始网站图片
 增加监测目标对象/主题,请 登录 直接在原文中划词!