Science - 16.08.2019

LiEt 3 BH increased to 0.5 ± 0.1 (fig. S11),
indicating that at this point the benefits due to
enhanced band-edge bleaching become over-
whelmed by detrimental acceleration of Auger
decay. Injection of extra electrons into cg-QD also
leads to a considerable reduction of the ASE thresh-
old (fig. S12 and supplementary text S9), as was
previously observed in ( 9 ).
To relate the observed lasing thresholds to QD
charging levels, we conduct measurements of
PL dynamics under the same conditions as those
used in the lasing experiments (fig. S13). These
measurements indicate that the use of 0.2 M
LiEt 3 BH results in ~2.5.Whentheamount
of LiEt 3 BH is increased to 0.4 M, becomes
close to ~3.1. As this corresponds to the lowest
lasing threshold, of ~3 likely represents the
optimal charging level for which the band-edge
absorption is strongly bleached (by nearly 90%),
whereas the fraction of heavily charged dots with
fast Auger recombination is still relatively low.
Numerical modeling of the lasing effect using the
gain model of Fig. 1 supports the assessment that
a dramatic reduction of the lasing threshold down
to the sub–single-exciton level is a direct conse-
quenceofthechangeintheopticalgainmech-
anism, i.e., a transition from the biexciton to the
charged-exciton gain (fig. S14).
Our experiments show that by using a charged-
exciton gain mechanism, it is possible to realize

strong lasing performance with sub–single-exciton

thresholds that compare favorably (in terms of

bothjp,lasand <Nlas>) with the lowest lasing

thresholds previously reported for colloidal-QD–

based devices (table S1). The advantageous effect

of charged-exciton gain is associated with both

the reduction of the gain threshold and lengthen-

ing of the gain lifetime. The benefits of a charging

approach can be realized with existing QDs and

previously developed lasing schemes, which

should facilitate its exploitation in future QD

lasing studies as well as its practical implemen-

tation in real-life lasing devices.

REFERENCES AND NOTES

1. I. D. W. Samuel, G. A. Turnbull,Chem. Rev. 107 , 1272– 1295
   (2007).


2. S. Yakuninet al.,Nat. Commun. 6 , 8056 (2015).


3. V. I. Klimovet al.,Science 290 ,314–317 (2000).


4. C. Danget al.,Nat. Nanotechnol. 7 , 335–339 (2012).


5. F. Fanet al.,Nature 544 ,75–79 (2017).


6. Y.-S. Park, W. K. Bae, T. Baker, J. Lim, V. I. Klimov,Nano Lett.
   15 , 7319–7328 (2015).


7. V. I. Klimov, A. A. Mikhailovsky, D. W. McBranch, C. A. Leatherdale,
   M. G. Bawendi,Science 287 , 1011–1013 (2000).


8. J. Lim, Y.-S. Park, V. I. Klimov,Nat. Mater. 17 ,42–49 (2018).


9. K. Wu, Y.-S. Park, J. Lim, V. I. Klimov,Nat. Nanotechnol. 12 ,
   1140 – 1147 (2017).


10. C. Wang, B. L. Wehrenberg, C. Y. Woo, P. Guyot-Sionnest,
    J. Phys. Chem. B 108 , 9027–9031 (2004).


11. G. E. Cragg, A. L. Efros,Nano Lett. 10 , 313–317 (2010).


12. See the supplementary materials.


13. W. K. Kohet al.,Sci. Rep. 3 , 2004 (2013).
    14. K. Wu, J. Lim, V. I. Klimov,ACS Nano 11 , 8437–8447 (2017).
    15. J. D. Rinehart, A. M. Schimpf, A. L. Weaver, A. W. Cohn,
    D. R. Gamelin,J. Am. Chem. Soc. 135 , 18782–18785 (2013).
    16. W. van der Stamet al.,Chem. Mater. 30 ,8052– 8061
    (2018).
    17. G.Morthier, P. Vankwikelberge,Handbook of Distributed
    Feedback Laser Diodes(Artech House, ed. 2, 2013).

ACKNOWLEDGMENTS

We thank Z. Robinson for assistance with modeling of electronic

wave functions.Funding:The studies of charged QD photophysical

properties were supported by the Solar Photochemistry Program

of the Chemical Sciences, Biosciences and Geosciences Division,

Office of Basic Energy Sciences, Office of Science, U.S. Department

of Energy. The lasing studies were supported by the LDRD program

at Los Alamos National Laboratory.Author contributions:V.I.K.

conceived the idea. I.F. synthesized the cg-QDs and performed

TEM measurements. O.V.K. conducted photocharging experiments

and optical gain measurements. T.N. performed TEM and AFM

measurements. J.R. designed and fabricated DFB cavities. Y.-S.P.

conducted lasing experiments. V.I.K and O.V.K. wrote the

manuscript with input from the other authors.Competing

interests:The authors declare no competing interests.Data and

materials availability:All data are available in the main text or the

supplementary materials.

SUPPLEMENTARY MATERIALS

science.sciencemag.org/content/365/6454/672/suppl/DC1

Materials and Methods

Supplementary Text

Figs. S1 to S14

Table S1

References ( 18 – 20 )

15 March 2019; accepted 12 July 2019

10.1126/science.aax3489

Kozlovet al.,Science 365 , 672–675 (2019) 16 August 2019 4of4

RESEARCH | REPORT