This work reports the first double p-extended undecabenzo[7]helicene 1,w hich is al arge chiral nanographene,composed of 65 fused rings and 186 conjugated carbon atoms.T he molecular identity of 1 has been confirmed by single crystal X-rayd iffraction. Aw ine coloured solution of 1 in dichloromethane absorbs light from ultraviolet to the near infrared, featuring an extremely large molar absorption coefficient of 844 000 M À1 cm À1 at 573 nm. Optically pure 1 shows ar ecordh igh electronic circular dichroism intensity in the visible spectral range (j De j= 1375 M À1 cm À1 at 430 nm) knownf or any discrete polycyclic aromatic hydrocarbon. These unusual photophysical properties of 1 contrast sharply with those of amono-undecabenzo[7]helicene derivative 2.

A high average ZT value (ZTave) of ~ 1.13 in n-type PbSe-based thermoelectric material at 300-873 K have been achieved in this work. Its high thermoelectric performance originates from ultrahigh...

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coefficient, σ represents electrical conductivity, T represents working temperature in Kelvin, κ ele denotes electronic thermal conductivity, and κ lat denotes lattice thermal conductivity. [4,5] Extensive efforts are devoted to decoupling these correlative parameters. [6] Electrically, the strategies of band convergence, [2] band alignment, [4,7] densityof-states (DOS) distortion, [8,9] modulation doping, [10] enhancing the symmetry of crystal [11] and quantum confinement [12] are successfully established to equilibrate the Seebeck coefficient (S) and the electrical conductivity (σ) in favor of a superior power factor (PF). [13] It is well known that the enhanced band degeneracy due to band convergence could increase the effective mass a bit without degrading the carrier mobility. And DOS distortion, which improves the band effective mass, is a feasible strategy with risks for deteriorating the carrier mobility. [14] It is clear that the optimal balance between the effective mass and the carrier mobility is beneficial for the electrical performance of TE materials. This relationship primarily depends on the weighted mobility, µW = μ H (m * /m e ) 3/2 , where μ H represents the carrier mobility, m * represents the DOS effective mass, and m e represents the unit electron mass.Thermally, intensifying the phonon scattering is considered to be an effective method to decrease the thermal conductivity (κ lat ), which is generally classified into incorporating extra phonon scattering centers [15] and seeking inherent low lattice thermal conductivity materials. [16,17] The latter representing materials might have a complex crystal structure, [1] heavy constituent elements, [18] intense lattice anharmonicity, [19] and soft chemical bonding. [20] And the extra phonon scattering sources include point defects, nanoprecipitates, grain boundaries, and so on. [21] To date, state-of-the-art TE materials, including Zintl phase, [22] half-Heusler, [23] skutterudite, [24] SiGe, [25] chalcogenides, [8,26] and Bi 2 Te 3 -based compounds, [27] etc., exhibit prominent thermoelectric performance.GeTe is proven to be an eminent mid-temperature thermoelectric material. [28,29] The well-recognized characters of GeTe are multiple valance bands, phase transition, ultrahigh carrier concentration, and high thermal conductivity, which diversify the degrees of freedom to tailor its TE performance. [30][31][32] Counter-doping using aliovalent elements, such as Bi, Sb, and In, [33,34] is a common method to achieve the optimal carrier Thermoelectric materials can achieve the direct conversion between electricity and heat, which has drawn extensive attention in recent decades. Understanding the chemical nature of band structure and microstructure is essential to boost the thermoelectric performance of given materials. Herein, CdSe alloying promotes the evolution of multiple valence bands in GeTe, resulting in the contemporaneous appearance of band convergence and density of state distortion, which benefits the sharply enhanced effective mass from ...

Thermoelectric generators enable the conversion of waste heat to electricity, which is an effective way to alleviate the global energy crisis. However, the inefficiency of thermoelectric materials is the main obstacle for realizing their widespread applications and thus developing materials with high thermoelectric performance is urgent. Here we show that multiple valence bands and strong phonon scattering can be realized simultaneously in p-type PbSe through the incorporation of AgInSe2. The multiple valleys enable large weighted mobility, indicating enhanced electrical properties. Abundant nano-scale precipitates and dislocations result in strong phonon scattering and thus ultralow lattice thermal conductivity. Consequently, we achieve an exceptional ZT of ~ 1.9 at 873 K in p-type PbSe. This work demonstrates that a combination of band manipulation and microstructure engineering can be realized by tuning the composition, which is expected to be a general strategy for improving the thermoelectric performance in bulk materials.