All entries for vitor manfrinato

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Controlled Placement of Colloidal Quantum Dots in Sub-15-nm Clusters

• Authors: V. R. Manfrinato, D. Wanger, D. B. Strasfeld, H. Han, F. Marsili, J. Arrieta, T. Mentzel, M. G. Bawendi, K. K. Berggren
• Sponsorship: EFRC Center for Excitonics

Semiconductor colloidal quantum dots (QDs) are electronically quantized systems with promising applications in optoelectronic devices^[1]. A key aspect of such systems is the fine control of optical properties in the synthesis process^[2]. These QDs are predominantly used in thin-film arrangement, deposited by spin casting or dip coating. Single QD patterning is one of the major challenges to designing a system that takes advantage of individual properties of QDs^[3]. Here we present a templated self-assembly technique to control the position of individual QDs through electron-beam lithography (EBL). This optimized top-down lithographic process is a step towards the integration of individual QDs in optoelectronics systems for industrial applications.

The fabrication process for QD placement occurred in four steps. First, a poly(methylmethacrylate) (PMMA) resist was spin coated on a silicon substrate to a thickness of 12 nm. Then, EBL was performed to obtain an array of holes (templates). The QD solution (6-nm-diameter CdSe or 5-nm-diameter CdSe/CdZnS) was spin cast or drop cast on top of the PMMA holes, and the remaining resist was removed by dissolution in acetone for 3 min^[4]. This process resulted in QD clusters attached to the substrate, as shown in Figure 1. For the application of patterned QDs in excitonic or nano-optical devices, optical characterization is required. We investigated the resilience of the photoluminescence (PL) following the patterning process. The samples were observed with confocal scanning microscopy. Figure 2 shows PL signal of a sub-15-nm diameter QD cluster. Thus, the placed QD clusters may be used for further experiments and applications.



Figure 1: (a) Scanning-electron micrograph of lithographically placed 6-nm-diameter CdSe QDs. The QD clusters were fabricated using PMMA holes with 8-15 nm diameter. (b) Histogram of the number of QDs in each cluster versus the number of clusters, for the sample in (a). We analyzed 54 sites designed for QD clusters. The QDs were counted from the SEM micrographs. Representative SEM images were added to the histogram.



Figure 2: Photoluminescence time trace of a sub-15-nm diameter CdSe/CdZnS QD cluster that was placed by this method. The time trace shows intermittent luminescence (blinking). The inset shows PL intensity versus frequency. The PL intensity presented a bi modal distribution, indicating a blinking state of the placed QD cluster.




1. A. P. Alivisatos, “Semiconductor Clusters, Nanocrystals, and Quantum Dots,” Science, vol. 271, no. 5251, pp. 933-937, (1996). [↩]
2. S. A. Empedocles, D. J. Norris, and M. G. Bawendi, “Photoluminescence Spectroscopy of Single CdSe Nanocrystallite Quantum Dots,” Phys. Rev. Lett. vol. 77, pp. 3873-3876, (1996). [↩]
3. J. Liddle, Y. Cui, and P. A. Alivisatos, “Lithographically directed self-assembly of nanostructures,” Journal of Vacuum Science & Technology B, vol. 22, pp. 3409-3414 (2004). [↩]
4. V. R. Manfrinato, D. D. Wanger, D. B. Strasfeld, H. Han, F. Marsili, J. P. Arrieta, T. S. Mentzel, M. G. Bawendi, and K. K. Berggren, “Controlled placement of colloidal quantum dots in sub-15-nm clusters,” Nanotechnology, vol. 24, 125302 (2013). [↩]

Resolution Limits of Electron-beam Lithography toward the Atomic Scale

• Authors: V. R. Manfrinato, L. Zhang, D. Su, H. Duan, H. G. Hobbs, E. A. Stach, K. K. Berggren
• Sponsorship: EFRC Center for Excitonics DE-SC0001088, Brookhaven National Laboratory

Electron-beam lithography (EBL) readily enables the fabrication of sub-10-nm features^[1]. However, the resolution limits of this technique at length scales smaller than 10 nm are not well understood. The known resolution-limiting factors of EBL are electron scattering, spot size, development process, and resist structure. We minimized the influence of electron scattering in our process by using 200-kV accelerated electrons. To minimize the spot size, we chose an aberration-corrected scanning transmission electron microscope (STEM) with 0.14-nm spot size as the exposure tool. STEM exposures at 200 keV have been done in conventional resists before^[2],^[3], resulting in feature sizes of 6 nm and resolution (i.e., pattern half period) of 15-nm half-pitch. However, the resolution-limiting factors were not systematically explored. In this work we did STEM exposures in hydrogen silsesquioxane (HSQ) at 200 keV. We then developed the structures with salty development ((J.. K. W. Yang and K. K. Berggren, “Using high-contrast salty development of hydrogen silsesquioxane for sub-10-nm half-pitch lithography,” Journal of Vacuum Science & Technology B, vol. 25, no. 6, pp. 2025-2029, Dec. 2007.)) and performed bright-field TEM metrology^[4].

Figure 1 shows maximum resolution of 5-nm half-pitch and 2-nm feature size. The reduced spot size in the STEM was responsible for the minimum feature size achieved. To compare the resolution limit of this technique with others, we measured the point-spread function (PSF) by using single-pixel exposures and measuring the feature radii as a function of dosage. The radii from zero to 2 nm were not achieved experimentally. Thus from the structural shape shown in Figure 1D, we back-calculated the 200 keV PSF from zero to 20 nm radii, which we called “calculated PSF”^[5]. We used the PSFs (measured and calculated) to calculate the energy-density contrast for a given pitch, as shown in Figure 2. The confined deposition of energy density at 200 keV translates into higher energy-density contrast, which was critical for achieving a higher resolution at 200 than at 30 keV.



Figure 1: TEM micrographs of 10-nm-thick HSQ on 10-nm-thick SiNx membrane substrate, exposed at 200 keV. (A) 10-nm-half-pitch HSQ dot array with 5.1± 0.8 nm feature size. (B) 5-nm-half-pitch HSQ dot array with 5.6 ± 1.2 nm feature size. The dose was (A) 18 and (B) 6 fC/dot. (C) and (D) are typical ~2 nm feature size with 8 nC/cm linear dose.



Figure 2: Calculated energy-density contrast, K(p)=(Emax-Emin)/(Emax+Emin), for the fabrication of two adjacent posts at 30 and 200 keV exposures. The blue and red curves were calculated using the fitted PSF for 30 and 200 keV, respectively. The black curve is based on the calculated PSF, predicting higher resolution than using the fitted PSF at 200 keV.




1. J.. K. W. Yang and K. K. Berggren, “Using high-contrast salty development of hydrogen silsesquioxane for sub-10-nm half-pitch lithography,” Journal of Vacuum Science & Technology B, vol. 25, no. 6, pp. 2025-2029, Dec. 2007. [↩]
2. C. Vieu, F. Carcenac, A. Pépin, Y. Chen, M. Mejias, A. Lebib, L. Manin-Ferlazzo, L. Couraud, and H. Launois, “Electron beam lithography: Resolution limits and applications,” Applied Surface Science, vol. 164, pp. 111-117, Aug. 2000. [↩]
3. S. Yasin, D. G. Hasko, and F. Carecenac, “Nanolithography using ultrasonically assisted development of calixarene negative electron beam resist,” Journal of Vacuum Science & Technology B, vol. 19, no. 1, pp. 311-313, Jan. 2001. [↩]
4. H. Duan, V. R. Manfrinato, J. K. W. Yang, D. Winston, B. M. Cord, and K. K. Berggren, “Metrology for electron-beam lithography and resist contrast at the sub-10-nm scale,” Journal of Vacuum Science & Technology B, vol. 28, no. 6, pp. C6H11-C6H17, Dec. 2010. [↩]
5. V. R. Manfrinato, L. Zhang, D. Su, H. Duan, R. G. Hobbs, E. A. Stach, and K. K. Berggren, “Resolution limits of electron-beam lithography toward the atomic scale,” Nano Letters, vol. 13, pp. 1555–155, 2013. [↩]