Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 - August 2, 201

Gunawan, A.A.

Mkhoyan, A.

Norris, D.J.

Thomas, M.G.

Wills, A.

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Imaging ‘Invisible’ Dopant Atoms in Semiconductor Nanocrystals A. A. Gunawan1, A. Wills1, M.G., Thomas2, D. J. Norris3, K. A. Mkhoyan1 1Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455 2Applied and Engineering Physics, Cornell University, Ithaca, NY 4853 3Department of Mechanical and Process Engineering ETH Zürich, 8006 Zürich, Switzerland  Incorporation of impurities, or dopant atoms, into semiconducting materials is a critical part in optimizing the performance of electronic devices. In nanoscale semiconductors such as nanocrystals (NCs), statistical fluctuations in the number and position of the dopant atoms can have a dramatic effect on the electronic and optical properties of the NCs. Hence, it is important to locate the position of dopant atoms. Annular dark-field scanning transmission electron microscopy is a powerful technique for such purpose, as demonstrated previously on various combinations of dopant atoms and host materials [1, 2]. However, those experiments require that the difference in atomic numbers between the host crystals and dopant atoms (ǻZ) be sufficiently large to yield a visible contrast in the image.   In this study, we utilized a spectrum imaging technique to simultaneously record electron energy loss spectra (EELS) and annular dark field – scanning transmission electron microscopy (ADF-STEM) images in order to identify Mn dopant atoms inside ZnSe NCs [3]. This enables dopant detection to be performed on a system of host crystals and dopant atoms which does not have a large ǻZ. This technique can be an alternative method in identifying dopant atoms inside materials whereby conventional ADF-STEM imaging is difficult to perform due to low ǻZ.   The ZnSe NCs were synthesized using high temperature organometallic solution based route [4]. The Mn doping was achieved by injecting dimethylmanganese into a mixture containing diethylzinc and Se. The number of incorporated Mn atoms can be tuned by varying the concentration of dimethylmanganese. Three types of samples were examined: (1) 2.9 nm diameter NCs with an average of 0.7 incorporated Mn atom per NC (singly doped), (2) 3.7 nm NCs with an average of 6.2 Mn atoms per NC (highly doped), and (3) 2 nm undoped NCs used for reference.   The spectrum imaging was performed using Nion aberration corrected Ultra-STEM at Cornell University. The accelerating voltage (100 kV) and beam current (~150 pA) were adjusted to minimize beam damage on the samples while still maintain a large signal to noise ratio. The resulting spectra were stored in 3-D matrices corresponding to the spatial (x, y) and energy loss data (z) and were filtered to remove counting noise. Mn L2,3-edge spectrum from bulk Mn samples were used to perform a linear-least-square (LLS) fitting for each of the Mn L2,3-edge signals from the NCs. A two-dimensional EELS map (M(i,j)) was then obtained following a relation M(i,j) = G(i,j)S(i,j) where G and S represent the goodness of fit and intensity scaling factor respectively. The presence of Mn dopant atoms corresponding to a pixel is characterized by large values of G and S in M(i,j) which was spatially correlated with the ADF-STEM image through the overlay process shown in Fig. 1a. This overlaid image is used to locate the presence of Mn dopant atoms in the NCs. In the case of highly doped NCs (sample 2) consistent detection of Mn is obtained as shown in Fig. 1b.  298doi:10.1017/S1431927612003340 Microsc. Microanal. 18 (Suppl 2), 2012© Microscopy Society of America 2012https://doi.org/10.1017/S1431927612003340Downloaded from https:/www.cambridge.org/core. University of Basel Library, on 30 May 2017 at 20:33:53, subject to the Cambridge Core terms of use, available at https:/www.cambridge.org/core/terms.The localto estimaelectron results in[111] Zntop surfaarises froMn atomof the do    Referenc[1] P. M.[2] A. M[] A. A.[] D. J. N[] E. J. K Fig. 1. (aEELS maZnSe NCfor (b) Mno Mn EFig. 2. (athe [100]image of surface. (STEM im beam intente the expebeam as it  the strongeSe NC is shce. The simum the 1.3 nms, however, ped column es:  Voyles et aittal, K. A. M Gunawan eorris et al.irkland, Ad).  ADF-STEp for the Ms (sample 2n-doped ZnELS signals) Calculated, [110], and a 4 nm ZnSc) Overlap oage for the sity of the Scted EELS propagates st beam inteown in Fig. lation conf deep Mn care undetec(circled) arel., Nature, 4khoyan, Ut al., Nano L, Nano Lettevanced ComM image on L2,3-edge a). (c) OverlaSe NCs (bot, as expected STEM elec[111] directe NC with twf the simulacase in (b) uTEM probesignals fromthrough the nsity along 2c whereby irms the detorrespondintable using  indistingui16 (2002) 8ltramicroscoetters, 11 (2rs., 1 (2001)puting in Ef Mn-dopedlong with thp of the Mnh shown in . tron beam inions in ZnSeo Mn dopated normalinder the ex was calcula the Mn atNCs. The the [111] ditwo Mn atoectability ofg to the peasole ADF-Sshable from26.  py 111 (201011) 5553. 3. lectron Mic ZnSe NCs (e correspon L2,3-edge in(b)) and (d) tensity at th NCs. (b) Snts (circled)zed Mn L2,3perimental cted using Moms [5]. Fiarrangementrection. Thems are locat both Mn atk in the simTEM imagin the adjacen1) 1101. roscopy, 2ndsample 1). (ding ADF-Stensity mapundoped Zne atomic coimulated hig located 1.3-edge intensonditions uultislice simg. 2a shows of atoms i simulated ed 1.3 and 2oms with thulated beamg (Fig. 2b) t columns.  edition, Spb) The extrTEM imag and the ADSe NCs (samlumn as it ph resolution and 2.9 nmity map witsed in Fig. 1ulations in  intensity on each direM(i,j) for a .9 nm belowe stronger s intensity. Tas the intenringer (201 acted core-lee of a Mn-dF-STEM imple 3) showropagates al ADF-STE below the th the ADF-b. order f the ction 4 nm  the ignal hese sities 0). vel oped age ing ong M op Microsc. Microanal. 18 (Suppl 2), 2012 299https://doi.org/10.1017/S1431927612003340Downloaded from https:/www.cambridge.org/core. University of Basel Library, on 30 May 2017 at 20:33:53, subject to the Cambridge Core terms of use, available at https:/www.cambridge.org/core/terms.

Imaging ‘Invisible' Dopant Atoms in Semiconductor Nanocrystals

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