Direct band gap silicon quantum dots achieved via electronegative capping

Abstract

Direct bandgap silicon quantum dots achieved via electronegative capping Poddubny, A.N.; Newell, K. General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. We propose a theoretical concept of switching between direct and indirect band gap character in silicon quantum dots (SiQDs) by the use of surface potential induced by the ligands or environment in which SiQDs are immersed-both cases are studied. Theoretical simulations show that the density of states of confined electrons in both real and k space can be dramatically altered by engineering the local electrostatic field. Especially interesting is modification of the lowest excited states, which appear in the valley for electronegative field that "pulls" electrons towards the SiQD surface. Opposite sign of the field does not have such effect at all. Hence we conclude a general trend of promotion of directlike radiative transitions by electronegative capping/environment. The rates are enhanced by more than two orders of magnitude compared to "normal" SiQDs, which can be as high as the values characteristic for direct band gap semiconductors. This model is in agreement with observed experimental properties of SiQDs with covalently bonded electronegative ligands.

Silicon Nanocrystals: From Indirect to Direct Bandgap

“…Tensile strain then cooperates with quantum confinement by shifting both the points in the same direction, ultimately leading the fundamental bandgap cross‐over . The second explanation of direct‐bandgap luminescence transition then lies in the electronegativity of the C‐linked surface . The direct transitions are a direct consequence of the surface capping, since the replacement of oxide surface with organic capping leads to fast PL and vice versa .…”

“…[42] The second explanation of direct-bandgap luminescence transition then lies in the electronegativity of the Clinked surface. [45] The direct transitions are a direct consequence of the surface capping, since the replacement of oxide surface with organic capping leads to fast PL [46] and vice versa. [41] The fundamental direct transitions in Figure 3 are a major leap forward, because they put the R rad of these NC on a par with other direct-bandgap materials.…”

Моделирование Методом Сильной Связи Кремниевых И Германиевых Нанокристаллов О Б З О Р

“…Строгий расчeт электрон-ных состояний таких нанокристаллов методом сильной связи невозможен, так как этот подход не позволя-ет учесть перестройку химических связей. Однако в работах А.Н Поддубного c соавторами [39,67,68] на основе метода сильной связи были получены качествен-ные результаты, показывающие возможность управления оптическими свойствами кремниевых нанокристаллов. В работах [39,67] кремниевый нанокристалл, покрытый радикалами CH 3 −CH 3 −, моделировался как нанокри-сталл, покрытый одним слоем углерода.…”

“…В ходе дальнейшего исследова-ния была выдвинута следующая идея: переключение ха-рактера запрещенной энергетической щели кремниевых квантовых точек в состояние с прямыми оптическими переходами определяется просто тем, что радикалы CH 3 −CH 3 − являются электроотрицательными и это вызываeт перетекание электронной плотности к границе нанокристалла. Эта идея была развита в работе [68], где были представлены результаты моделирования, по-казывающие возможность переключения при пассиви-…”

Bright trions in direct-bandgap silicon nanocrystals revealed by low-temperature single-nanocrystal spectroscopy

“…The use of SiNCs instead of the bulk form of silicon, however, can alleviate the problem of poor light emission only partly, because SiNCs preserve the indirect nature of the bandgap 1,2 , and the light-emission rate thus remains low, on the order of 10 4 s 21 or less 3 . However, when passivated by a "suitable" organic material, SiNCs can have fast macroscopic radiative lifetime [4][5][6][7] and can actually be transformed into a direct-bandgap material 5,[8][9][10] . In particular, we have recently shown both theoretically and experimentally that properly capped SiNCs can be strain-engineered into a material with fundamental direct C 15 -C 25 ' bandgap 5 and the corresponding fast electron-hole radiative recombination rate of 10 8 s 21 (see also Figure 1a and the section on "Results overview").…”