Two-dimensional <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi mathvariant="normal">M</mml:mi> <mml:mn>2</mml:mn> </mml:msub> <mml:msub> <mml:mi mathvariant="normal">X</mml:mi> <mml:mn>2</mml:mn> </mml:msub> <mml:mo> </m

The exploration for novel two-dimensional (2D) monolayers with diverse electronic characteristics has attracted growing interest in recent years. Using density functional theory (DFT) calculations, we have predicted a new family of 2D transition-metal- (TM) based compounds under the nomenclature <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:msub> <a:mi mathvariant="normal">M</a:mi> <a:mn>2</a:mn> </a:msub> <a:msub> <a:mi mathvariant="normal">X</a:mi> <a:mn>2</a:mn> </a:msub> </a:mrow> </a:math> (where M represents TMs and X denotes chalcogen elements like S, Se, and Te). Our investigation delves into the examination of the formation energies, dynamical or thermal stabilities, mechanical properties, electronic structures, and magnetic properties of various systems within this family. Through our computational analyses, we have discovered a total of 35 thermodynamically and dynamically stable <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:mrow> <d:msub> <d:mi mathvariant="normal">M</d:mi> <d:mn>2</d:mn> </d:msub> <d:msub> <d:mi mathvariant="normal">X</d:mi> <d:mn>2</d:mn> </d:msub> </d:mrow> </d:math> monolayer materials that exhibit remarkable diversity in terms of their electronic and magnetic properties. Our findings will pave the way for the experimental realization of various <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"> <g:mrow> <g:msub> <g:mi mathvariant="normal">M</g:mi> <g:mn>2</g:mn> </g:msub> <g:msub> <g:mi mathvariant="normal">X</g:mi> <g:mn>2</g:mn> </g:msub> </g:mrow> </g:math> structures in the near future. In particular, among the predicted compounds, <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"> <j:mrow> <j:msub> <j:mi mathvariant="normal">M</j:mi> <j:mn>2</j:mn> </j:msub> <j:msub> <j:mi mathvariant="normal">X</j:mi> <j:mn>2</j:mn> </j:msub> </j:mrow> </j:math> <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"> <m:mo>(</m:mo> <m:mi mathvariant="normal">M</m:mi> <m:mo>=</m:mo> <m:mi>Zn</m:mi> <m:mo>,</m:mo> <m:mo> </m:mo> <m:mi>Cd</m:mi> <m:mtext>;</m:mtext> <m:mo> </m:mo> <m:mi mathvariant="normal">X</m:mi> <m:mo>=</m:mo> <m:mi mathvariant="normal">S</m:mi> <m:mo>,</m:mo> <m:mo> </m:mo> <m:mi>Se</m:mi> <m:mo>,</m:mo> <m:mo> </m:mo> <m:mi>Te</m:mi> <m:mo>)</m:mo> </m:math> are a direct band-gap semiconductor with band gaps between 0.9 to 2.6 eV (1.3 to 3.7 eV) by DFT+Perdew-Burke-Ernzerhof (hybrid functional HSE) calculations. <q:math xmlns:q="http://www.w3.org/1998/Math/MathML"> <q:mrow> <q:msub> <q:mi mathvariant="normal">M</q:mi> <q:mn>2</q:mn> </q:msub> <q:msub> <q:mi mathvariant="normal">X</q:mi> <q:mn>2</q:mn> </q:msub> </q:mrow> </q:math> <t:math xmlns:t="http://www.w3.org/1998/Math/MathML"> <t:mo>(</t:mo> <t:mi mathvariant="normal">M</t:mi> <t:mo>=</t:mo> <t:mi>Ti</t:mi> <t:mo>,</t:mo> <t:mo> </t:mo> <t:mi>Zr</t:mi> <t:mo>,</t:mo> <t:mo> </t:mo> <t:mi>Hf</t:mi> <t:mo>,</t:mo> <t:mo> </t:mo> <t:mi>Tc</t:mi> <t:mo>,</t:mo> <t:mo> </t:mo> <t:mi>Re</t:mi> <t:mo>)</t:mo> </t:math> are zero-gap semiconductor (semimetals) in standard <v:math xmlns:v="http://www.w3.org/1998/Math/MathML"> <v:mrow> <v:mi>DFT</v:mi> <v:mo>+</v:mo> <v:mi>PBE</v:mi> </v:mrow> </v:math> calculation. Inclusion of spin-orbit coupling leads to a gap opening of 0.1 eV. Notably, our analysis has also unveiled the magnetic nature of certain materials, such as <w:math xmlns:w="http://www.w3.org/1998/Math/MathML"> <w:mrow> <w:msub> <w:mi>Mn</w:mi> <w:mn>2</w:mn> </w:msub> <w:msub> <w:mi mathvariant="normal">X</w:mi> <w:mn>2</w:mn> </w:msub> </w:mrow> </w:math> <y:math xmlns:y="http://www.w3.org/1998/Math/MathML"> <y:mo>(</y:mo> <y:mi mathvariant="normal">X</y:mi> <y:mo>=</y:mo> <y:mi mathvariant="normal">S</y:mi> <y:mo>,</y:mo> <y:mo> </y:mo> <y:mi>Se</y:mi> <y:mo>)</y:mo> </y:math> , <bb:math xmlns:bb="http://www.w3.org/1998/Math/MathML"> <bb:mrow> <bb:msub> <bb:mi>Fe</bb:mi> <bb:mn>2</bb:mn> </bb:msub> <bb:msub> <bb:mi mathvariant="normal">X</bb:mi> <bb:mn>2</bb:mn> </bb:msub> </bb:mrow> </bb:math> <db:math xmlns:db="http://www.w3.org/1998/Math/MathML"> <db:mo>(</db:mo> <db:mi mathvariant="normal">X</db:mi> <db:mo>=</db:mo> <db:mi>Se</db:mi> <db:mo>,</db:mo> <db:mo> </db:mo> <db:mi>Te</db:mi> <db:mo>)</db:mo> </db:math> , and <fb:math xmlns:fb="http://www.w3.org/1998/Math/MathML"> <fb:mrow> <fb:msub> <fb:mi>Ti</fb:mi> <fb:mn>2</fb:mn> </fb:msub> <fb:msub> <fb:mi>Te</fb:mi> <fb:mn>2</fb:mn> </fb:msub> </fb:mrow> </fb:math> . The prediction of semiconducting (magnetic) <gb:math xmlns:gb="http://www.w3.org/1998/Math/MathML"> <gb:mrow> <gb:msub> <gb:mi mathvariant="normal">M</gb:mi> <gb:mn>2</gb:mn> </gb:msub> <gb:msub> <gb:mi mathvariant="normal">X</gb:mi> <gb:mn>2</gb:mn> </gb:msub> </gb:mrow> </gb:math> materials not only offers valuable insights into the underlying electronic properties (magnetism) of 2D systems but also positions these materials as promising candidates for the development of advanced electronic (spintronic) devices.